The hero discovers that the one they love told them a lie and feel deeply hurt and betrayed, even if the lie was well-intentioned, usually causing an obstacle in the process. A perfect example is Princess Atta, who upon discovering that Flik lied to her about the Circus Bugs, she tells him to leave with them and not come back.
The hero discovers that the one they love did something horrible in their past and finds themselves unable to forgive them. A good example would be John Watson, when he discovered that his wife used to be an assassin for the CIA.
The hero and the one he or she loves are forced to break up either to save each other or to prevent something horrible from happening. Often times, this issue gets resolved and they reunite. An example is Miaka Yuki and Tamahome's relationship.
The villain plays a trap to make the hero and heroine break. An example of this, is when Dr. Mashirito building a false Arale for to mislead Obotchaman that she hated him and will leave the village, and so we can destroy Arale.
The lover was cheating on the hero's back. Often happens with a married couple.
The hero is rejected by their crush without an opportunity of a relationship, either because they are not interested or because they only see the hero as a friend. In the worst cases the crush can do it for hatred.
The hero's love rival won the heart of the love interest.
The heroes' lover dies it cause heartbreak for the loss of the love ones and swore vengeance toward the murderer when Yugao Uzuki's Hayate Gekkō was killed by Baki under the deception of Orochimaru.