I don't like people telling me to have a nice day. I will have a nice day if I want to have a nice day or I'll have a horrible day if I want to have a horrible day - it's my choice as to what sort of a day I have. I don't need anyone to tell me to have a nice day. I'll decide that for myself.
What would be preferable would be, "I wish you a nice day." Still not the best but rather better than telling me I have to have a nice day.
What do you think? Do you need someone telling you what sort of day you should have?