I think having no respect for the other, and not having the capacity to compromise.
Comparing or categorizing their relationship to that of others. And not fully understanding their love or the act of love itself
Assume too much too soon
Having kids early in the relationship/marriage. Really should wait 5+ years.
Having recently broken up with someone, the one factor amongst many was that I started to forget about my needs, I started to neglect the things that made me happy and started over-thinking and over investing too much time into him and the relationship ...you need to maintain a sense of self even when you are in a relationship nurturing your own needs also - keep doing the sports and activities you love, meet with your friends. Someone who loves you freely will be happy to see you happy and strong, be with someone who respects you being you and doing other things outside of the relationship
taking the loved one for granted
Thinking one owns their partner
they think they know each other. Altho I find most of teh women I have met thinking they know about Males-rarely are they right. I mean, women,..., on a consistent basis-how many guys have you really gotten to know-literally sat down and played 100 questions about how "he feels about things?. I do this with almost every woman/Female I date. If I didn't, well I was not interested in them. But I rarely think I have a Female figured out-they keep their knowledge close and I like that, their behavior proves what they know about Males-women for the most part will spout off all they think they know about Males/men with no real ideas of why they think it. It's a HUGE turn off when a Women throws all her preconceived notions about men on a Male, never wanting to indulge and really know a Male.
Taking the other for granted
SAYING "i love you" just to please the ear
Not listening, and all to often one side will assume something the wrong way.
Being snoopy. Looking through phones, and creeping on facebook. That creates distrust, and you cannot have a relationship without trust. It also makes you paranoid which just screws you in the long run.
Getting together with no info.
Sleeping with another person without protection and then coming back and sleeping with their mate like nothing happened. Goes both ways. You all know its true.
Losing their own identity.
You took the word right out of my mouth, dishonesty.
Yapping too much!
Being too possessive and jealous.
Having sex too soon. You get physically attached and then you find out you're not compatible somewhere else and somebody bails...