Why does my boyfriend look at random girls' photos on Facebook?

I hacked in to my boyfriends Facebook and I found he messaged lots of random girls whilst being in a relationship with me. We had a big bust up and since then I know his password. I have realized that he still is looking at random girls photos and I don't understand why he has to do that as I don't have the need to do that. I feel like he is searching for other girls on Facebook because he is not satisfied with me. Should I be concerned or am I just insecure? :(