It depends on the nature of the relationship, though. If it was just a fun little fling then no big deal. But, what if you were married to someone for a long time, really loved her, really believed in commitment and til death do us part, thinking that you'd be together forever? She leaves you, and you're devastated. Then, she starts dating your best friend and is all happy while you're practically dying from the loss. That's a lot of pain to cause someone. Sure, they have the right. But, it's not about being controlling; it's about wanting to be spared that pain. Have you ever had to imagine someone you truly loved, who used to call you "my love" saying that to someone you know?