Remus would like to forget everything about his past. He wants to forget Peter and James and especially Sirius because wouldn't it be easier that way? To have them erased from his mind? That way he couldn't replay in his mind scenes that seemed so perfect back then but now that he looks back on them, are really so tragically twisted and horrible.
They trusted Peter—look how that turned out. They trusted them with their lives, and he betrayed them oh so easily when something better was offered to him. It's funny, his animagus was a rat. They should have known.
And James, oh. Remus wants this grief and pain to go away nearly more than anything else in the world. He's lost James and he's lost Lily and he's lost so much more too, and he can't stop thinking about the time they all went to Majorca for a holiday or the time James told Remus that it's okay if you're a werewolf, we're still your friends or worse, the time Remus held Harry in his arms for the first time, James and Lily beaming at them and chatting happily. Sometimes it's still hard to believe that they're gone.
But most of all, he wants to forget Sirius. His laughter, his voice, his lips, his hands… It was so much easier back then. They were in love and nothing hurt. But now, like everyone else Remus has ever loved and trusted, Sirius is gone. Forever. And all Remus has left are the memories.