This topic could fill whole libraries to overflowing. There are all kinds of reasons. To start with a few simple ones though, some people actually view relationships merely as a means to a predatory lifestyle and only ever manipulate and abuse their partner.
Relationships that are not like this can fail due to lack of communication coupled with one or both parties becomeing bored or distressed somehow. Others might find that they are simply not the sort of people who can live together, even if it seemed they would be from being short term acquaintances.