Mutual support in each other's fulfilling their identity, each other's dreams and accomplishments, caring for each other's feelings leaving expression of this always open, being positive and non-competitive, not being critical, accepting each other's quirks, having a meeting of the minds, being affectionate, having each other's back, sharing goals, having lots of fun in bed, laughing a lot, of course, being honest with each other.....It matters that they should be equally intelligent and share interests.

ok like @atheist your question caught me short. I am here in part because I haven't learned the answer to that question. Oh, I could sprout lots of sage sounding advice and then doubt your intelligence if you took any notice of me. Sorry.

I think a successful (by which I mean strong, happy, loving) is a lot of hard work. It can be very difficult adjusting your life to mesh with someone else's. It requires dedication, consideration, respect, and equality. Liking your partner as well as loving them is also important. So are compromise, open-mindedness, and a willingness to really listen and consider the other person's thoughts and feelings. Trust is essential. Above all, it means always remembering that your is the most important person in your life.

I was going to answer your question but then I realized that all of my romantic relationships have failed. So I'll pass.

If you are more specific, I'll try to answer.