A few reasons, most of which are small, but build up to be pretty large.
First, he had very little faith in me. If I were to go for a uni exam and come out saying "That was harder than I'd expected!" he would automatically assume I'd failed, that we'd wasted money on my failure, and what was the point in trying again, I'd just fail again.
Second, I often felt as though my opinion was superfluous. When we were buying a house, I put forward my life savings that I'd been squirreling away since 15 or so. Despite objecting vociferously, he put an offer onto a shite house in an area well known to be controlled by a bikie gang, and basically said "sign the papers, or I'll sign them for you." At 21 I honestly thought he could do that.
He did the same thing with my car- refused me money to have it serviced because he didn't like it. Didn't care the tyres were bald, or that it wouldn't idle right. Blamed me when the police pulled me over and fined me. Apparently it was my fault for driving a shonky car. His motorbike, though, that got anything it needed.
Third, if he was going to be late, he would never, ever let me know. He worked night shift and so we didn't have dinner together, we had breakfast. I would time breakfast so that it would be ready ten minutes after he got home at 6am, except he would just...roll in the door at seven or eight and wonder why I was upset.
Fourth, I wasn't particularly good at expressing my point of view coherently. Everything came across as "because it's what you're meant to do!" and then I would cry because I was frustrated with myself for being incapable of being any clearer.
Fifth, he left his work boots in the hall for me to break myself on bringing the laundry in.
Six, he left his undies balled up in his pants, didn't empty his pockets, and left his socks balled up inside the other.
I woke him up one afternoon and just said "I want you to move out on Sunday. DBro is bringing his trailer to help." He just said "okay," and went back to sleep. That Sunday was quite literally the second best day of my life.