Carl is old and he builds new memories and shared his life with a child and a dog. Carl hates his companions.

The child he must suffer is simple and naive. The dog is plainly an idiot. But Russell is trying to improve himself and Dug is pure of heart.

The adventure and how these two creatures display virtue and improve his life in spite of being unable to compare to his late wife nor the more sophisticated and accomplished Muntz reveals to him that life is still worth living and that connections are still to be made.

I think it makes sense.