It is important to have trust in a relationship.
Mark's girlfriend of 4 years, Hannah, has plans to have a girls-night-out tonight with some friends.
Mark wants to have confidence Hannah isn't cheating on him.
Mark realizes Hannah hasn't cheated on him in 4 years and realizes she wouldn't begin to now.
Mark asks Hannah if she promises not to sleep around on him while she's out.
Hannah thinks Mark's trust of her is appealing, and she loves him more.
Hannah see's his jealousy as annoying and breaks up with him.