We don't want to disappoint or hurt our significant other and feel extremely guilty when we do. College goes by in the blink of an eye and long distance students find themselves having to pass on events out of fear of confrontation with their significant other or find themselves having to hide the truth a bit. They simply grow apart and develop new interests. When we are on our own, we find ways to compensate for losses in our life. This is not done out of spite, it is our most basic survival instincts kicking in.
Reunions are passionate.
Separations are emotional. This is like putting a Band-Aid on an amputation as it doesn't feed any aspect of the relationship except physical desire. Thus, the rest of the relationship doesn't get nurtured. We like to be around people who are healthy and who care for us. Our significant others from so far away cannot do these things for us.