Keep in mind your child KNOWS this and often takes advantage of it.
How do you balance loving with being tough? One important element is to not let the situation get so out of control that you are disciplining from a state of emotion. This means reacting with a punishment before your child gets the "power" to push your buttons.
Then you are disciplining from a logical - thought through - stance which will keep the emotion out of it and help you to stay in control.