Passionate love lasts for two or three years at most. After that, the relationship continues on the basis of friendship, life sharing and trust. A new partner is the only way to feel that high feelings again created by the passionate love. That's the main reason for cheating; consciously seeking this emotion or experiencing it by chance.

However, any newly started relationship will inevitably come to the same low standard level after a while. This is true for all relationships. Therefore, it does not make sense to make "the lack of this feeling" the reason for a break up.