You miss the contact, the smile, the attention of your loved on. You're rejected, a source of pleasure and companionship had been removed. And it hurts everytime we break up with someone. It can be motivated by a number of things. Curiosity about someone.
Some people are just immature or self-centered and feel entitled to sex with others.
There's a lot of reasons, probably as many as there are people. The task of a therapist is to help get control of this and stop the pain. Some like lots of kisses and hugs. Others like oral sex. Some just like to celebrate their partner's body.