Why do forces always exist in pairs

According to Newton's third law, for every action force there is an equal (in size) and opposite (in direction) reaction force. Forces always come in pairs - known as "action-reaction force pairs." When you push on an object, the object pushes back with an equal force.

For example:

Think of a pile of books on a table. The weight of the books exerts a downward force on the table. This is the action force. The table exerts an equal upward force on the books. This is the reaction force. Note that the two forces act on different objects. The action force acts on the table, and the reaction force acts on the books.