How to keep a relationship? Do some break-ups happen for no reason?

I have had two serious girlfriends, and both of them dumped me. The first one had a crush on me at first sight and chased me for a while before I realized she was, so I made a move. She was 24 and she was Japanese (I am French and we communicated in English.) We had a great relationship, very romantic and with deep feelings. She used not to like the fact that I was going to the gym because, according to her, it was one hour of the day we didn't spend together. She said there was nothing I could ever do that could make her angry at me. Well in four months she broke up with me because she didn't love me anymore and now she hates me (tried to get her back and be her friend too much).

With my second girlfriend, things were lighter, but we had deep feelings for each other. She was 22 and from Korea, we communicated in English as well. She lost her virginity to me, she told me several times that to her I was the best, and later on she told me she loved me (after I said it earlier in the relationship)... but she broke up with me three weeks after she said that. Our relationship lasted only two months in total. At first when she asked for space and said she wasn't sure if she still loved me, she told me several times that I didn't do anything wrong and that there was nothing I should change, that she just had so much on her mind with school issues that she needed time for herself and she also said she missed her single life. She said that in the relationship, she loved me deeply.

One month after she broke up she was sleeping with another guy (who was already posting on her Facebook since a little before we broke up), who is now her new boyfriend.

I just really don't understand, I am really trying my best to be a great boyfriend, and it seems to work so well at first, but then they break up with me. My two relationships lasted four months and two months... I know that sometimes people just break up because the feelings are not there anymore, but it normally takes a lot longer than that! I feel like there's something I'm missing but I don't know what.

So I have two questions: do you think that sometimes break-up happen for really no understandable reason at all? And do you have any advice as to how to keep a relationship?
How to keep a relationship? Do some break-ups happen for no reason?
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