Failure to perform or respond to friendship-maintenance tasks can lead to the deterioration and eventual dissolution of friendships. Causes of dissolution may be voluntary (termination due to conflict), involuntary (death of friendship partner), external (increased family or work commitments), or internal (decreased liking due to perceived lack of support) (Bleiszner & Adams, 1992).
Sometimes friendships can also deteriorate due to internal conflict on one side, or personal issues, I've had reasons to end relationships, while I also have been the one who has been theoretically "dumped" in a relationship.