There are a variety of reasons why this might happen.
1. They "relax" and take each other for granted. Stop putting in the same kind of effort that won the heart of their mate to begin with. Some people treat relationships and marriages like purchasing a sofa. Once they get it home they put it in a spot and leave there for 30 years.
However relationships are more like gardens. You have to nurture them or else they'll die. It's not uncommon to hear someone say, "He/she is not the same person I fell in love with."
2. Some people are suckers for the "infatuation phase" of relationships They're "in love" with "falling in love". Once both people reveal their "authentic selves", have a few major disagreements, and realize their mate is not "perfect" they begin to lose interest in them.
3. Growing apart instead of growing together overtime. When people stop wanting the same things for a relationship it's pretty much over. There is no amount of "work" or "communication" that can overcome being with someone who does not want what you want.
4. Someone is not having their emotional or physical needs met. This can often lead to cheating or a breakup.
5. In the eyes of one of them the other has committed a "deal breaker" which is unforgiveable. Love becomes hate and hate eventually becomes indifference.