It was a bunch of things over time. We married very young (I was 19 and he was 21), and were together for 20 years by the time we divorced. He changed a lot over time, and I didn’t much, so that was one big thing. I didn’t mind that he changed, but he minded that I didn’t.
Another factor was me going through therapy. I’d gone into it with the idea that it would help me be a different, better person. Instead, it helped me see that there wasn’t anything terribly wrong with the person I already was. When I accepted that, and was content with myself as-is, I was much happier but he saw it as a cop-out…that I was no longer trying to be ‘better’. In the end, we just made each other miserable. We decided we were too young to go on that way forever, and that it wasn’t the best environment for our children either, so we parted ways. Mostly amicably.