I love you, But or And?

I am one of those moms who has a list of phrases I repeat over and over. I think you know what I mean. "What did I just say to you?" "Attitude, people, attitude." "You carry your father's name." One of my common phrases really struck me as I heard myself tonight. "I love you, but..." Why would I say that. The context of the conversation dealt with a behavior I didn't want my child to continue. I think I said something similar to " I love you but I am not going to let you treat your brother that way."
Why would I say BUT ? Is love letting someone get by with bad behavior? I should love my children enough to correct them. Discipline is love! If I don't teach and discipline them, I set them up for a terrible life. I think my children sense that I hesitate to discipline them. I came from a home where the father was the primary disciplinarian. I am sometimes uncomfortable with that role. I have misplaced guilt because I know it is necessary and good yet I feel terrible that I have to do it. Sometimes I even feel like I am a poor mother because my children need discipline. How does that even make sense? The truth is that I love my children enough to correct them. When I use phrases like " I love you, but..." my children are going to interpret my meaning that discipline means I don't love them; that when I won't let them do what they want, it means I don't love them. I must stop. I will now say, " I love you AND I will not allow you to continue this behavior." Because I do love them more than any guilt or awkwardness I may feel about being the mean mommy. They are my little treasures.