I have had some casual relationships which didn't last longer than 1 month (I'm really not sure why I was with them tbh, probably just experimenting), and 2 serious relationships. Well, the first serious relationship lasted 3 years, but I wasn't completely happy, he didn't understand me and I continued to be in the relationship 2 years more than I wanted (I know that's a long time). In all of those relationships mentioned above, I always needed my own space, I couldn't understand why people in relationships wanted to be around each other all the time, my feelings weren't strong and they most definitely didn't understand me. I accepted the idea of being alone, which suited me fine tbh, as I didn't think anyone would understand me.
My second serious relationship is one which I am still in. This one is completely different (in a very good way)! We started being friends by talking about our eccentricities. He has aspie-like tendencies which might help explain why we get on so well. Currently, we've been together for 2 years, 1 month and 2 days. I can honestly say that I don't see this relationship ending and he doesn't either. We just understand each other! We have a lot in common and finally, we both like to be around each other 24-7; there's no need for alone time! I never thought that I'd find this, but I have! Of course we have the occasional argument/misunderstanding, that's natural and is bound to happen, but we always make up afterwards and everything's okay. I think they key to this relationship working is the fact that we understand each other and we've accepted each other's eccentricities (I love him for those quirks), which means that we can completely be ourselves and the other one doesn't mind.
I'm not saying that everyone will be happy in a relationship, as some may prefer to be alone, but I did think that I'd be happy in a relationship, and now I don't .