Why do men get bored of relationships so quickly?

My friends and I have all been with our boyfriend's for over a year and now all three guys have gotten colder, seem bored of us, and their libidos have gotten lower. I try everything, when I try to cook my boyfriend something he finds any tiny reason to complain I never complain when he cooks for me, I've been trying to dress cute, work out, always look my best but he can't even seem to be bothered to spend much time with me. He used to be THE cuddliest most affectionate / passionate person for the first year, so I still try to interact with him in the same way we always used to. He kisses me with tight lips like I'm his grandma and pushes me away when I hug him too long. I don't even know how to interact with him anymore.

I don't think he really even realises he's doing it but HOLY SHIT it hurts a lot It's like a punch to the stomach and I feel this rejection daily! I usually have to initiate sex and I have been super sexually frustrated lately, he enjoys it when we start but he never really seeks it out or starts it anymore. Last night we were touching each other and I after we came I asked if it was bad that I still wanted to have sex with him and he said "YES... just kidding?" and went to sleep. I try to be the best girlfriend I can be but I constantly get rejected by him and ignored and it's really making me depressed. I want to do really sexy things for him to keep him excited and turn him on but I am SO SCARED to because I get rejected constantly and I feel like such a loser when he shuts me down over and over. Then when I ask him if he wants to break up he cries and freaks out, so if he doesn't really want to be with me but he also doesn't want to be without me what does he want? I give him his space and I'm not needy ( when I am lonely I usually don't say anything until it's really hurting because I don't want to be annoying) I just want a normal relationship, I don't want anything more than what we had to begin with.
Why do men get bored of relationships so quickly?
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