When I was younger I didn't, then I met my DH, and he was a stickler for wearing seat belts. When we bought a P/U and a camper shell for camping, he installed a 3rd belt in the center of the front seat, (bench seat, of course, old truck) so we could take one of the kids with us in the truck. He wouldn't allow any of them to ride in between us until he got that belt installed.
We took a week-long camping/fishing trip, and his 12 y/o son went with us. On the way home, we had an accident. We went around a mountain curve, right into a herd of deer. DH was driving, and though he knew better, he hit the brake. It's instinct, sometimes it's wrong. We went into a spin, went off the road, and rolled off the embankment. The truck rolled over two or three times before it stopped, upside down, with us hanging upside down. We were able to carefully release the seat belts, careful to land on shoulders, and not on our heads. All three of us walked away from that with minor bruises, mostly from getting hit with thermoses, flashlights, and other items in the cab of the truck with us. Had we not been wearing seat belts, who knows what would have happened. I'm sure it would've been a lot worse than a few bruises.
A couple of years later, I was on my way to work late at night, and a teenage girl, who did not have the right-of-way, turned left right into me, we had a not-quite-head-on collision, driver's side corner to driver's side corner. My seat belt kept me from going through the windshield, though I did hit my head on the mirror. I had injuries, pretty bad ones, that occurred on impact, so they most likely would've been the same, but there would have been more, if I'd also gone flying.
I'm so glad DH was so insistent about the seat belts, and was insistent about me doing likewise, and I had developed the habit. I always wear mine now, and so do any passengers in my car. Nothing like a little personal experience to drive a point home.