Plain and simple, men and women are wired differently. I know this comes as a shock to some, but it's true.
Men and women value different things in relationships, just as they do in all other aspects of their lives. Security is a bigger deal to the average woman than it is to the average male. Most women feel more secure with one man to call theirs. It's a feeling of being provided for, sheltered, whatever way you choose to word it. Nothing weak or wrong about that.
Men on the other hand value stimulation more. That explains the hyperactive sex drive the average teenage boy thrives on. Men are more likely to become bored in a monogamous relationship, as they crave the variety of different sensory input. Again, nothing wrong or weak about it.
Also, it's seen as a control issue. Men resent having anyone possess any sort of control over them, and often interpret a committed relationship as a personal sacrifice. When their partner doesn't recognize that sacrifice (as they naturally would not; women do not approach the situation with the same mind set), they resent and start to claim things like "She's trying to change me too much." Women, meanwhile, see it more as an agreement. They hear their mate complaining about all the things he has "given up" to be with them, and resent that mindset equally as much as the males resent their complaints. It causes friction and discord, which is the opposite of security, which in turn leads to the woman lashing out at the man for his silliness.
Wiring. Plain and simple. No cure for it. A relationship is a series of compromises, and when the balance is too far off kilter either way, the result is strife.
My bill is in the mail.