For us, it's that we're extremely close friends (we were friends for some time before dating), we both have a tremendous respect for one another, we have the same (fairly dark) sense of humor, and when we found each other we both knew how lucky we were and both were willing to upend all of our previous individual plans for life in order to be together.

None of that is a reproducible formula for a perfect relationship, and honestly my personal track record wasn't great before finding my wife. Really the only advice I have is to find someone that's not just a significant other but a very close friend -- a lot of people say that, but not many truly have it -- and, when you find someone like that, put her (or him) before absolutely everything.