paper is made from wood pulp, which is short, matted strands of cellulose extracted from wood. (It can also be made from threads from torn up old rags.) In any case, the pulp is just a wet mass dried flat on screens. Unlike cloth, paper is made without any process of weaving to tie it together. So, when paper gets wet, it becomes pulp again, and there is nothing holding it together, and it tears easily.