Is it because: He cheated, She cheated, loss of interest, found someone better, got scared, he changed, she changed... What do you think?
Most Helpful Guy
Growing apart. It is perfectly reasonable to get into a relationship for the benefits of the "honey and roses" phase to both people. You don't always go on a first date thinking this will probably be your lifetime significant other. You don't always have sex thinking that any particular conditions are attached (be that on a first date or over time).
As you get to learn about the other person's needs, interests, and values, you can just realize that it isn't meant to carry on. If this happens I think the person who wants out has a responsibility to the other to explain what they have to do, to let the other person have some time to react and vent, and to ask questions etc to try to get closure.
If the person ending it doesn't respect the soon-to-be-ex enough to do this, the other classic breakup behaviours may be used (cheating, "sudden" silence, and gutless stuff like dumping by text etc).1