Why do babies love to drop stuff over and over?

Around 6 months, babies realize that things exist even they can’t see them, a concept is known as object permanence. You’ll know it’s kicked in when the baby looks down to see what he has dropped.
It is a very fun game that teaches the baby the relationship between cause and effect. So, although you are tired to pick up things, continue playing with your baby. It is important for him.