I'd been mostly away from doing my own wrenching while I raised a family... just getting back to it a bit now, at 54.
Back in the day, there were several factors. First, my father was hopeless with tools, so I was the family fixit guy. Out on my own, I was lacking in funds but had a taste for dragging home mechanical orphans. Ever try to find someone to inexpensively work on a Renault Dauphine, MGC-GT, or Triumph TR250?
The motivations were curiosity, poverty, and the sense of accomplishment.
I got started by buying the largest wrench set Sears made, and the shop manual for the car in question. I was slow, methodical - did everything by the book and took Polaroids (digital wasn't invented yet) of every step of the disassembly. Replaced whatever needed it, and reversed the process. Then moved on to something bigger.
What helped me most was living in an industrial area, and being friends with my neighbors. I had an auto repair shop in my backyard; they'd let me use their lift and (very occasionally, with great suspicion) a tool. Across the street was a body shop...
Another big help was having lots of cars. Even though I was single, I'd keep 3 - 5 cars around, so if I got stuck with one all apart (or busted something irreparably), it was no big deal. These days, I'm single again and have two Passats - and I'm looking for a nice Corrado.