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most guys are deceptive in the beginning hiding most of their flaws and once they feel secure enough, like they have the girl, their real traits start to unravel and they become unattracted to the guy. They do this because they realize that it'll be hard for them to find another girl.
You pretty much answered your own question here. It's not always the fault of the guy though. Believe it or not, women will subtlety try to manipulate you into revealing these flaws, and potentially cause you to develop flaws you didn't have in the first place. It's called betaization.
It's talked about on the sosauve forums until the thread title "How could it have gone wrong??" - The Sh*t Test Everyone Fails. They refer to it as an active shit test, and not a process like betaization, but it's essentially the same thing.