Mine are:
1. Its not a hobby. Its almost a religion ( I hope I don't offend anyone). Its spiritual and I dream and daydream the stuff.
2. I enjoy it a lot.(Most of the time)
3. It saves money. No matter what a new car's average price is 27K now.
4. I am challenged by it.
5. It keeps my mind young and it keeps me young.
6. The business aspect is neat. I get to buy tools, too. Its fun shopping around for used parts and tools.
7. Its in keeping with my Zen of life. I'm frugal, like to understand what makes things work (people, now thats a whole 'nother ball game, bro!) and like to restore and fix things.
8. I take pride in it
9. I love the sound and smell of a diesel.

other thoughts:
a. I am glad I don't have to do it for a living ( No disrespect to those who do.) Thats because, one, I am not good enough and two, I dont think I could handle the customer relations end which is actually very tough and requires good business and people skills.
b. I always wonder, why not Real estate or stocks, something I coulda made a pile on. I dunno, money is everything and yet it isnt.