By definition, a relationship involves two people who entered into an agreement with each other, explicitly.
It is the characteristic of humans to be bored. The mind seeks to be entertained. If a "perfect" partner exists, that too is a concept.
The challenges or problems even within such relationships where both people love each other very much are an inherent part of being in it.
Happiness, without any problems, in any relationship is temporary. It is the continued commitment of each other to stay together that is important, for it gives three things: peace, emotional security and fearlessness. It is possible..when both love each other unconditionally, the implicit assumption being "You are free to leave, at will. I love you anyway and wish you to be happy."
Only when one can say that beyond the shadow of any doubt can happiness be possible for long periods of time.