Cooper loves playing fetch so much he sleeps with his ball

His ball goes with him everywhere. He has 3 green tennis balls and 1 rubber ball and likes them equally. We play in the house in any room. We play outside in the backyard. If I'm in the office, he'll drop it at my feet. If I'm sitting on the sofa he'll drop it in my lap. When I come home from being out he greets me with the ball in his mouth. He'll even want me to toss it across the bed if we're in bed.
When we are outside though he'll never drop it. I have to pull it out of his mouth.
Max doesn't really play ball. I've seen him retrieve it once or twice. But he really enjoys stealing it from Cooper when Cooper drops it on the sofa for me