The easiest relationships I've had (non romantic) are with people I've known since I was a child. Most of them I don't talk to regularly anymore, we've all changed over the years, but when I needed them the most they were there for me.
The easiest romantic relationship was were we had been friends for years and I wasn't physically attracted to him but it was a nice and comfortable existence.
The hardest would be the person I was with that led me here.
It's been a while since I've had a romantic relationship of note but I'd say they tend to be long term. If/when something goes wrong I'm inclined to assume it's my own fault and to exhaust those around me. Last night I was exhausted and beating myself up over things while also taking it out on a very good friend of mine. I mentioned that I felt as though people were sick of me and my problems and he said "I'll be honest, you're hard to talk to when you get down on yourself. Like nothing anyone could say would make a difference." I argued that they were just words and he said "It doesn't matter that they don't hold any weight. You have people that care about you, deeply"
To me that sums up my problems with relationships. My gullible nature has bitten me in the ass so many times that it's hard to take anything at face value anymore.