Firstly, I like the feel of moving heavy iron against gravity.
That in itself isn't enough, though. I've been fat, unfit and over forty and can't let the first two take hold of me ever again. I've had a wasted and useless posterior chain and it's not acceptable.
If ever you let doubt slip into your mind about whether you want to be the man-mountain of muscle that you work to become, just brush against one of those soft, pillowy worms that pass for people around you. Assuming you don't vomit, hold that sensation. Can you imagine yourself ever wanting to be like that?
For me, it has little to do with vanity. I do it to be healthy and to feel healthy and strong.