When my so was that age, he did the exact same thing. One day he said, "Mommy, I love you with enough love to fill the earth and sky, but I love DADDY with enough love to fill the earth and the sky and the rocks and the trees, and the grass and the dirt!" I never let it bother me (I thought it was cute). I knew that he loved me, but I kind of created the situation myself. Whenever Daddy would come home, I would excitedly point out that daddy was home. It would be a big deal to see him. I was with him all day long, and there was no one to "throw a party" of get excited when I walked into the room. There will be phases when dad is the favorite and phases when mom is the favorite.