This is a pretty interesting topic actually.
For some people, it's really hard to tell when something has to change for the better. Some people are so used to being someone or doing something over and over that changing that routine is hard for them. Especially if it's an awful habit or addiction. So if a person has shitty habits, they're far more like to rationalize them and perpetuate them than to actually fix those mistakes. That can escalate in a relationship as now it's not just you being the victim of your own shitty habits, it's also your partner. And of course, they can have an entirely different perspective on how a husband/wife acts compared to yours.
The point I'm trying to make is that sometimes people really do act like dicks but they don't intend to do so and that spreads into a relationship.
Now in a relationship, you should obviously talk with a partner about it and if you're willing to do stuff like therapy and visiting doctors/psychiatrists and such for the better, then yeah. It's good to change. You'd want to give your partner the best possible life and I can't see why you'd want to keep harming a person you supposedly love over and over. So big changes that affect your psychology and physically are things I would definitely change to help benefit my partner.
However, small things like hobbies or likes/dislikes are off the chopping block. When you get into a relationship, you get into it KNOWING (hopefully) who you're with. If your partner likes to play games a lot be expected to see that throughout your relationship. It's one thing if your partner has destructive habits. It's another when it doesn't harm you whatsoever.