Why does my boyfriend keep looking at his ex's facebook pics?

My boyfriend and I are pretty serious & living together. We share my laptop since he doesn't have one & I've found on my internet history that he has been looking at his ex's facebook and her facebook pictures. Why? Should I be worried? Should I ask him about it? There was an incident when she kept calling and texting and he told her to stop & he has moved on. But why keep looking at her facebook & pictures on facebook? It has been twice in the past 3 days.