What do you think is important for relationships to work out long term?

I am happy to hear anecdotes as well as evidence based stuff.

There are stories where people say love fades, sex fades, but the relationship still goes strong for a long long time.

But, the thing is what make you fall in love with someone? To choose them as your life partner? What kind of assurance does one need to be totally free and secure mentally and physically in front of (with) one another?