If you're asking what factors sustain relationships, some of the main ones are:
1/ good communication
2/ the ability to compromise
4/ being able to forgive and be flexible
5/ having more things in common than not (beliefs, habits, moral views, etc)
Although you hear that opposites attract, when it comes to long lasting healthy relationships, studies show that the two partners have to have more things in common.
If that was not what you were asking, feel free to clarify your question.