yup...how would you know?..hmm...i'll attempt to explain, but for everyone it's different...
IMHO when your past the honeymoon stage of the relationship and are still able to make each other happy, be there for each other, etc etc...also, the things you do that keep the relationship happy are done w/o effort...to do it because you believe it's required is more of an "obligatory attitude" than a "i'm doing this because i want to" attitude...and that's just skimming the surface, but as said, it's different for everyone and explanations can go on forever...