Hope...friend or enemy?

Why do we hang on so tightly to people that aren't really good for us?  I guess everyone is different, but some of us just have a much harder time letting go.  Whether it be old boyfriends, a man/woman that did us wrong, a one sided friendship .... there is nothing to gain from the toxicity!  The only thing I can figure is that those of us who do it are just clinging to hope.  Hope that one day it will be better or different; that somehow the person or the dynamics of the relationship will change.  I've always thought of hope as a good thing; it is what has gotten me through so many rough situations in my life.  But the older I get the more I realize that hope has a very ugly side.  Hope leads me down paths I shouldn't take.  Hope smiles so warm and invitingly, yet makes promises that it just can't keep. I am a fool.  I desperately cling to the toxicity because I stupidly have hope.