While you just know that they love you, for some they just aren't sure since as parents we must sometimes be hard on our children. Children often think of our discipline methods as being mean and not caring which leads them to think we don't love them when it's actually the opposite.
Children at a young age don't realize that we discipline and correct them because we love them. That is something children don't learn til they get older, usually once they become adults. This is part of the reason we must verbalize our love and then they in turn will learn that it's okay to say that they love their parents and others in life as they develop.
Also in religion (as a Christian anyway) we are taught that God want us to love all regardless of their faults. Also 'love' is very natural and can be expressed in many ways and mean many different things to different people.