I guess Cooper thought Ben was cold, and maybe he needed a hug or a toddler to ride him.
I can't believe how lucky we are to have a dog like Ben. For all his idiosyncrasies, he tolerates a lot from Cooper. Pay careful attention to Ben's head and expression. It didn't change once throughout this entire episode. Yes, he looks pitiful, but I think he doesn't mind so much.
Oh, and don't worry Mom, I washed the blanket before Cooper slept with it.