Why does it always happen?

There are all types of people out there so it is hard to generalize. You can't generalize in the world at all, but there are a few things you can take in to consideration. From my experience through friends and the like, I've found that many who have the same interest such as in anime/manga/comics/video games, etc are rather introverted. This is not just a phase for a few, but mostly due to inexperience in social interaction. Yes, it is hard to make new friends and introduce yourself. Can you do it? Yes. Many people feel a sense of security behind the internet or phone. Whatever device you choose, there is no replacement for a face to face talk or actually meeting the person. Technology only goes so far. We are social creatures and need that interaction in order to really get a grasp of what someone might be like. Actions speak louder than words. Mind you, this is all just my opinion, nothing more. As for your issue, there is no couple out there that doesn't go through the 'puppy love' phase of a potential relationship. This phase can last anywhere from a few days to maybe even years in some cases that I've seen. 'Puppy love' is the whole "I like this person and don't understand them fully yet" mentality. Time can change this to "I actually love this person" or "This person isn't the one I'm interested in". Variety is the spice of life and is one of the best ways to keep a relationship. Excuse my wall of text.

I think age is an important consideration, but mental soundness as well. A difference of two or four years isn't insurmountable. It's a matter of mental maturity (on both sides). Do you want the same things? Do you have the same expectations?

Usually its me that just kinda accepts anything I find wrong. Like one guy, he was my bestfriend. He smoked, but I dealt with it because it was him. >_> I don't have high expectations. I let big things just brush off and yet, they find one thing wrong with me, I'm the one who's in trouble.

it's difficult to say what the problem is when i have no facts or details...but i think you should take a step back and really look at your past without being biased. the problem just might be you. and nobody likes a doormat. and the people who do like doormats shouldnt be in your life anyways. if there's something you dont like then say it. and if they decided to leave, then you're both better off.