Good question! Honestly I never considered that piece of headcanon until now so here is my initial guess:

Assuming he got his fanciful degree in psychology, he wanted to do something good with it. A job that would be practical and allows him plenty of mind room to work around complex situations. In other words: The less papers he's pushing the better. And joining in a field where you can prevent possible harm towards innocent people seems right up his alley.

OR if we want to be ridonkulous about it- his muddah told him too. 8Ib