Two people involved in a relationship make a choice to be together, but it only takes one of them to decide to end the relationship. Even more tragic, death can occur and no one gets to make the decision. This is the reason relationship endings are sad, and it happens on a fairly regular basis.
There are times when a relationship has just begun, and one of those involved suddenly realizes they have more important goals. They leave their new partner to pursue them, but their personal excitement overwhelms their sense of sadness. Other relationships last for years, but a major life change may cause it to suddenly crumble. Whether the relationship is long or short, death cuts it off like a knife.