Sorry but these types of questions 'why did you become a therapist' are pretty common and pretty tame. And relevant to the therapeutic work. Personally, I would be wary of a therapist who would refuse to answer this...

It's not like you asked him a personal / private life question, which I *would* understand if he refused to answer.

If after YEARS of seeing him he won't answer such a tame question, the shame ought to be on him (for making you feel this way).