Yep, fingers and thumbs seem to take a lot of abuse! I've noticed my left seems to get more than my right as I'm right handed and the left is the "holder", and the right the "abuser"...
The one I can't quite figure out is the time (I was maybe 5 or 6?) I put my left middle finger through a bike chain and gear. My older brother had my bike upside down, and had spun the pedals as fast as he could. (This bike was locked up in the rear hub so the pedals were spinning with he wheel after he let go) I decided that was bad for the bike, and tried to catch up with the spinning pedal with my left hand and then grabbed for it.
The bike also had no chain guard and my middle finger got caught in the spinning gear, and made the loop. It nearly severed the tip of the finger at the base of the nail. it was still on just by a corner.
Off to the hospital, where they cleaned it up, said it could not be stitched, (hamburger..) and simply pushed it back in place and bandaged it with LOTS of gauze to keep it there and said to see it it took and grew back on...
It actually did! That gauze "splint" was changed periodically, but was on there for several weeks as I recall. I'm certain the bone had been severed/broken/mangled as well. I had no feeling in the tip for a number of years, and the scar still has minimal if any feeling. Most of the tip regained feeling, and after being "shed" the nail even grew back. That finger is now about an eight of an inch longer than the same finger on the right, and the nail has to be trimmed far down the side on the side that was severed. Overall I was lucky.
Buy WHY did I grab with my left? 
I'm VERY right handed. Almost awkward with my left doing delicate or intricate things. Was I left handed then, and the trauma made me switch?

Did the trauma to my left make me stop using it unless forced to?

I'll probably never know...