So many relationships are broken entirely in your head.
We often only see our weaknesses when they're reflected in others. And that's not just rhetoric; it's actually science. People do it all the time, and not just in relationships. How many times are we going to see an anti-gay politician later outed as homosexual?
Looking inward and addressing your own fears and doubts is one of the most difficult things to do. It takes immense strength and courage to do something like overcome generations of cultural and religious condemnation to admit you're gay.
But it's much easier to attack an existential threat in others. The same goes for relationships. Why is it so often that the jealous, controlling type is the one that winds up cheating? Instead of addressing their own distressing thoughts and behavior, they seek out and attack any sign of it in their partner.
This goes for any insecurity. If you're convinced your partner thinks you're fat, despite a continued sex life and their insistence otherwise, it might be because you're insecure about your own body.
The problem with not recognizing when you're projecting your insecurities onto someone else is that they become a sort of self-fulfilling prophecy.
Take someone you love unconditionally, like one of your parents. Tell them you hate them one time, and they might shrug it off. Tell them 100 times, and eventually they'll start to believe you, even if it isn't necessarily true.
Now, take your partner. You're worried that they don't really love you, even though they say that they do. If you ask them, "Do you really love me?" enough, eventually they might wonder, "Wait, do I?"
It's basically a simple, psychological mindf*ck. If you repeat something enough times, people start to believe you.
If the thing you're repeating is merely a projection of your insecurity, congratulations! You just willed the thing you were worried about into existence. Creating problems out of thin air and imagination is most definitely not the recipe for relationship success.