I mostly blame society because our culture has advanced so much and we meet so many people so fast. Another reason is we just don't have the time to sit around and relax anymore. My adjustment teacher said, "we work so much, that when we finally retire, we don't even know the person we married anymore." People change so much over time, I have to agree that we just have no choice but to either move on or find the time to get to know our spouse again.
There is just too much going on anymore, that we are often forced away from the person we previously loved.