Honestly, I've been a different person in every relationship in my life. Different people bring out different things in us, some people make us want to be better people, some people inspire only our worst traits. It is usually the relationship where you know you have too much power over the relationship that inspires our nastiest side. So don't assume that power. Don't let it get a foothold. Don't be with people who would accept that from you. Every new relationship has all the potential in the world, for it to be good, and for us to fail at it.
I think the worst relationships are those where you both go in expecting the other to 'fix' everything wrong in your life, they fail, and you get mad. You fail, and they get mad. Then you both sit there, feeling angry and betrayed.
I really thought I was marrying my best friend. But, oh well. I could point out all his faults (mental illness, pathological lying etc) but the bottom line is that I wanted something so badly, that I was stupid and I made myself overlook the bad. I put on blinders and dove right in.
Not fair to anyone.
Ah, guess I grew up.