Why is it so hard to leave even when you have no reason to stay?

I am not sure if I am a believer in love , I just know that with every bone in my body I am trying to believe because without that glimpse of hope a world would be a really bad place to live in.
I can't quite think of when did the selfishness begin among people, but it has really made me disappointed in human race.
And here is what is bothering me, although I choose to believe in love ,I also know for a fact that most of people are selfish and I don't believe that love can mix with selfishness.
Where does that leave us?
Are we all stuck in a relationships where one of us gives more than the other and is it that one person is always starving while the other one is fulfilling every wish they have not giving a shit about the other person?
And if so how can we believe in that "I love you" when you know for a fact that they love themselves more?
How can you live on this planet and be your own favorite person,knowing everything you've ever done?
How can you care only for what you wish for ?
And then for the other person... How can you stay knowing this?
How come we feel so lonely while with someone?
Isn't that the worst feeling of all?
And why do we put up with it?
Last couple of days every time I get upset I remind myself that I can walk away and never deal with that crap again,but why do I stay?
Why do I make a victim out of myself?
If 80 percent of the other person drives you insane and hurts you deeply and if you with all of that see that they don't even try to get better ,than why are we still here?
Is it a hope?
Hope that it will change , that it will get better?
Doesn't that makes us stupid and insane ?
Because even though I want to believe in love I think that staying somewhere that brings you pain must be insanity.
We hold onto this hope because we don't want to be alone, but it seems that we are alone all the time.
And at the end of everything... The only person I get to blame is myself... I am the one choosing to hope for better despite all the facts , I am the one choosing to believe in change despite the fact that everything is fucking the same... I am the one choosing to hurt instead of walking out... So tell me....
If it hurts to leave and it hurts to stay...
What do you do?
Do you keep hoping that something will change ?
Why is it so hard to leave when you have no reason to stay?