I think its because they have passion. Me... i love computers. I would guess that the passion I feel when i am mucking about with a computer would be similar to the passion that a T feels when talking about feelings with clients. And then they would also get a "hit" of passion whenever something important happens, eg a client has some kind of breakthrough. That "hit" would last them through the down times, I bet, and then eventually they get another "hit" of passion from the next breakthrough or realisation.

I know drug references are bad but that was the only word I could come up with that gets the meaning across, lol.