Why do men leave their partners?

Why do men leave their partners? The common scripts you widely hear from men is “I need space”, “The problem is on me”, “I want to search myself”, “You don’t deserve me” or “You’re not the one I used to know”. I said scripts because they are untrue to a certain extent. Scripts, for they are just been said to lessen the pain and to give respect even for the last time. One of the true reasons I can think of is weariness or boredom. Men are sometimes selfish, they cannot help themselves to look for what they want inside their shabby relationship. They are looking for a perfect partner, it could be another woman, adventure, or career. This perfect partner will not only accept him as he is fully, but also will finally make him express the parts of himself that are hidden. She'll bring out the best in him. Rather than criticize and make demands, the perfect partner will give unconditionally and fulfill his every need. But you have to understand that this is only men’s fantasy. If only women could fully recognized this, they’d be able to know how to handle and deal with it.
I know a girl who became overweight while her husband is still slim and tidy. She’s finding reasons on how to defend her situation or why she was resulted in being fat, like she gave birth to their children, she doesn’t have time to exercise and etc. But when we are together, she most likely wanted to go out, check some restaurants and shop. Even at breakfast, while her mouth is still full, she’s already asking everyone what they can get for snack. Could you imagine that?!, she’s still eating her breakfast and then she’s planning ahead for having a snack. There are a lot of things to think of, but for her, it’s always food. I was not surprised when she told me that her husband consumes a lot of his time in doing something like reading, surfing the net, or fixing his car. I can’t blame him, it’s better to get into things that interests him rather than focusing to some unlikely sights. c",) This will make men more bored in the relationship, you see, what men and women don’t realize is that their true need is to find adventures in their relationship. This girl that I’m talking about should be challenged on how she can get back the attention of her husband, not only in the physical sense but also in finding ways to somehow fulfill his fantasies.