How come men are sweet only at the start of a relationship?

Well, my boyfriend used to be soooooooo sweet to me but after awhile, he changed. I felt bad because I wanted a sweet guy who always made me special so I asked him why he was like that. He said that he was running out of sweet things to say. So I told him that it's okay if he repeats some of the sweet things that he tells me. So what we did was, he sent me a goodnight text every night and tells me sweet things :) This went on for a month but he hasn't sent me anything for the last 4 days.

I just want a happy relationship with him and I'm afraid that if we stop being sweet with each other, we might get bored with each other! I don't want that to happen :(

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And I also feel like I'm too clingy. we talk everyday but every time he's busy with school, he becomes boring to talk to. It's his exam week in the university and he's busy. Me on the other hand, doesn't have anything to do because school is over :D
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Yeah and I feel neglected when he's busy. I'm shy to demand that he should tell me "I LOVE YOU" all the time (cuz it makes me feel better) so I kind of pout when he's paying more attention to his work. I understand that he's busy but I really miss him :(
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Thanks so much for your answers! :)
How come men are sweet only at the start of a relationship?
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