Why do some men behave this way?

Oh, you're dating one of those guys!  I'm a guy, but I'm not one of those guys.  But I know how those guys think, and I know how they work.  I know lots of those guys, and I know you don't want to stay in a relationship with any of them.  Not for a year, not for a month, not for fifteen minutes.

Those guys are exactly the same as those girls, only with deeper voices and (most times) more body hair.  They're the jealous types, and they're bad news every time.  They're also always the first ones to cheat, just so you know.

Those guys look at all the things that attracted them to you, like your makeup and hair and wardrobe, and they literally think you looked like that just to attract them.  When you keep looking like that, they think you're trying to attract someone else.  They think (I'm not kidding, they really think this) now that you've got someone, you should make yourself look unavailable to others as a sign of respect.  Because (in their minds) if you still look good, it can only mean you're still looking.  Again, I'm not kidding, they really think that.  

Now you know why.  And now you know why to avoid anything even resembling a relationship with them.

Maybe at first when you met, he was being more polite and complimented you to be nice. He didn't feel comfortable telling you exactly what he thought of your clothes & makeup at that time. Then after dating for awhile, he felt comfortable to tell you exactly how he feels or maybe realized he truly didn't like it after all after seeing more of your clothes & makeup over time.

Why stick around to listen to his insults?  Make sure he eats your dust. He's nasty

They think you may be attractive to other guys.

never been in a relationship as guy so i couldn't really tell you but all i can say is most women accidentally choose the asshole type of guy confusing him for being confident. and thus treat you this way.

"Some men" prefer their conquest to continue to be attractive but not to continue to look like she's on the market. And "some men" are attracted and horny but don't consider dating a gal to be their stamp of total approval. "Some men" are endlessly vying for power and feel that putting down their woman is a way of achieving power over her. "Some men" are trying to get the gal to break up with them with little effort on their part. And "some men" are just endlessly trying to "fix and repair", "Nip and tuck" and take on a gal like a project. "Some men" will do/say almost anything to get laid. 

Guys come with all sorts of behaviors and motivations.