More thorough answer: Yagyuu has Dad Issues. The way I play him, a large part of Yagyuu's determination to present the sort of socially perfect upstanding "Gentleman" image that he does is because of pressure, direct or implicit, from his dad at home. His dad is a doctor, whom I headcanon to work long hours at the hospital/clinic/etc because he is a Devoted and Hardworking Man and Provider For His Family, and Yagyuu's got a mom and a younger sister which I think ended up casting him as this sort of Man Of The House from a very young age. He wanted to be like his dad, he missed his dad because of how comparatively little he saw of him, and when he did see his dad, his father was instilling pretty strong family and social values — be a man, work hard, do this, do that, behave this way, etc.
The problem is, Yagyuu is smart, kind of introverted, sort of a loner, and can have the most fucking ridiculous determinator-martyr complex the world has ever seen. He feels a lot of pressure to be the Perfect Son, and a lot of times he gets frustrated because he feels as though his dad's already got his whole life planned out for him because of it. He'll get through school, go to med school, become a doctor, work at the same hospital as his dad, carry on the family tradition, marry a nice girl, have kids of his own, perpetuate the cycle. Bam. He's fourteen and he already knows how the entire rest of his life is supposed to go, and he feels trapped and resentful and miserable about it.
BUT. That's not actually the question. So golf; Yagyuu picked that up from his dad as a way of being like him and giving them something they could do to spend time together in his dad's rare leisure moments. They'd go golfing together. It was cool. Equally awesome is the fact that Rikkaidai Fuzoku has a nationally-ranked golf team, so he could join the golf club, earn a great line on his resume, after school extracurriculars, all that shit.
Somewhere along the way he discovered tennis, possibly because of Yukimura and how good he was. Yukimura kind of has that Jesus-like ability to inspire the soul of tennis in people. Regardless, Yagyuu got sort of interested in tennis, tried it out, turned out to be pretty good at it, this became a thing. Also not really a problem because Rikkai's tennis club was nationally-ranked, too.
The problem arose when Yagyuu went home and was discussing over the dinner table his inability to choose between the two clubs, because his dad was under the impression that it was always destined to be Golf All The Way, but now here was this tennis business and what's all this about. But Yagyuu's mom could see that Yagyuu was developing this interest in tennis, and he'd spent so much of his life suppressing his own interests and the stuff he wanted for the sake of what his dad thought was right anyway, and moreover Little Yagyuu didn't really have that many friends (if any) and his mom was entertaining this secret hope that maybe if he were on a tennis team, that'd be more of a team-oriented atmosphere than being the loner on a golf course, and maybe Hiroshi would make a friend or two and it would be good for him. Because she worries.
So his mom encouraged him to pick tennis club over golf club, and Yagyuu really really kind of wanted to play tennis, so he got up his courage and went out for it, and apparently ended up being really good, since canon states that of the seven third-year regulars we see out of Rikkai during the series, only four of them were regulars in their second year as well. Three of them were, of course, the Big Three, who'd been regulars since they were first years because they're just that boss.
But Yagyuu was canonically a regular in his second year of tennis club, and he's the only one of the remainder (Niou, Marui, and Jackal) who was. Which is pretty impressive! Given the fuss over Akaya being the second-year ace, it's kind of a big deal at Rikkai to be a regular and not in your third year. So he's pretty boss.
As additional commentary, he started out playing singles because he liked the thought of it better (one man against one opponent striving for victory), but given his favored playing style, he's objectively a better doubles player than he is at singles. Niou is a good match for him because Niou is a flashy, tricky attention-grab for their opponents, who tend to spend so much time worrying about OH GOD WHAT IS NIOU UP TO that they sort of forget that Yagyuu's just chilling at the baseline, waiting for the chance ball that'll let him get off his Laser — his favorite method of point-scoring, and the most reliable one in his arsenal. Yagyuu would be happy to just sit at the baseline and hit Lasers all day, and with Niou drawing attention off him during doubles, he gets a lot of opportunities to do exactly that.
no subject
Short answer: his mom and golf.
More thorough answer: Yagyuu has Dad Issues. The way I play him, a large part of Yagyuu's determination to present the sort of socially perfect upstanding "Gentleman" image that he does is because of pressure, direct or implicit, from his dad at home. His dad is a doctor, whom I headcanon to work long hours at the hospital/clinic/etc because he is a Devoted and Hardworking Man and Provider For His Family, and Yagyuu's got a mom and a younger sister which I think ended up casting him as this sort of Man Of The House from a very young age. He wanted to be like his dad, he missed his dad because of how comparatively little he saw of him, and when he did see his dad, his father was instilling pretty strong family and social values — be a man, work hard, do this, do that, behave this way, etc.
The problem is, Yagyuu is smart, kind of introverted, sort of a loner, and can have the most fucking ridiculous determinator-martyr complex the world has ever seen. He feels a lot of pressure to be the Perfect Son, and a lot of times he gets frustrated because he feels as though his dad's already got his whole life planned out for him because of it. He'll get through school, go to med school, become a doctor, work at the same hospital as his dad, carry on the family tradition, marry a nice girl, have kids of his own, perpetuate the cycle. Bam. He's fourteen and he already knows how the entire rest of his life is supposed to go, and he feels trapped and resentful and miserable about it.
BUT. That's not actually the question. So golf; Yagyuu picked that up from his dad as a way of being like him and giving them something they could do to spend time together in his dad's rare leisure moments. They'd go golfing together. It was cool. Equally awesome is the fact that Rikkaidai Fuzoku has a nationally-ranked golf team, so he could join the golf club, earn a great line on his resume, after school extracurriculars, all that shit.
Somewhere along the way he discovered tennis, possibly because of Yukimura and how good he was. Yukimura kind of has that Jesus-like ability to inspire the soul of tennis in people. Regardless, Yagyuu got sort of interested in tennis, tried it out, turned out to be pretty good at it, this became a thing. Also not really a problem because Rikkai's tennis club was nationally-ranked, too.
The problem arose when Yagyuu went home and was discussing over the dinner table his inability to choose between the two clubs, because his dad was under the impression that it was always destined to be Golf All The Way, but now here was this tennis business and what's all this about. But Yagyuu's mom could see that Yagyuu was developing this interest in tennis, and he'd spent so much of his life suppressing his own interests and the stuff he wanted for the sake of what his dad thought was right anyway, and moreover Little Yagyuu didn't really have that many friends (if any) and his mom was entertaining this secret hope that maybe if he were on a tennis team, that'd be more of a team-oriented atmosphere than being the loner on a golf course, and maybe Hiroshi would make a friend or two and it would be good for him. Because she worries.
So his mom encouraged him to pick tennis club over golf club, and Yagyuu really really kind of wanted to play tennis, so he got up his courage and went out for it, and apparently ended up being really good, since canon states that of the seven third-year regulars we see out of Rikkai during the series, only four of them were regulars in their second year as well. Three of them were, of course, the Big Three, who'd been regulars since they were first years because they're just that boss.
But Yagyuu was canonically a regular in his second year of tennis club, and he's the only one of the remainder (Niou, Marui, and Jackal) who was. Which is pretty impressive! Given the fuss over Akaya being the second-year ace, it's kind of a big deal at Rikkai to be a regular and not in your third year. So he's pretty boss.
As additional commentary, he started out playing singles because he liked the thought of it better (one man against one opponent striving for victory), but given his favored playing style, he's objectively a better doubles player than he is at singles. Niou is a good match for him because Niou is a flashy, tricky attention-grab for their opponents, who tend to spend so much time worrying about OH GOD WHAT IS NIOU UP TO that they sort of forget that Yagyuu's just chilling at the baseline, waiting for the chance ball that'll let him get off his Laser — his favorite method of point-scoring, and the most reliable one in his arsenal. Yagyuu would be happy to just sit at the baseline and hit Lasers all day, and with Niou drawing attention off him during doubles, he gets a lot of opportunities to do exactly that.