It’s hard to address a subject like this one. Worldwide, there are people who have coupled with someone they are displeased with for one reason or another. Yes, we adore one another during the whirlwind romance. We rip each others clothes off whilst in the throes of passion. And then domestic life settles in. We eat more than we should. Laze around on weekends more than we should. We drink too much, eat too much rich (and yummy) food. But when you choose a partner, you choose them for who they are and who they aren’t. The scary thing is that you probably have only a tiny inkling of who they are. It's a foundational lesson to grasp that when you are in a relationship, this does not give you the right to decide how the other person should be. If you have chosen someone who doesn’t suit you, that is your own damn fault. So my first advice would be to take great care in making a lifelong commitment with someone!
But apart from this, I think we need to seriously consider what we see as “acceptable” and “desirable” in a person. We happen to live in a society that worships physical beauty and doesn’t value the real and true things about a person. We compare our mates to others. We compare ourselves to others! In fact, I think this is the real root of the problem. A person who is constantly critical of their mate is likely overly critical of themselves. And they are fucking pissed off that their mate is comfortable with their flaws! "How dare they! I walk in misery, constantly striving for unattainable perfection, and they just lie there sleeping well through the night? Why aren’t they up, trying to improve themselves?" It’s a sick world, I tell you.
The fact is that if you have a healthy love for self, you are on the right track. First, you must be a good mate in order to find a good mate. Then, you must be reasonable about the person you choose to spend time with. And why you choose them. If you like spending time with one another, sharing mutual respect and consideration, that’s the large part of it. Of course, there needs to be attraction... chemistry. But chemistry is unreliable. It can’t be depended on to last through the years because it involves too many variables. And things change. We change. We all do. This is not a bad thing. I once heard that a woman marries a man expecting him to change and a man marries a woman expecting her not to. And they are both doomed to disappointment. So, let's just try not being a dumbass about it. Mind your own body. It's the only one that belongs to you.