As Zuko and Sokka was on their way to the Boiling Rock, they had this conversation.
Sokka: Do you miss anyone from the fire nation?
Zuko: I have a girlfriend, Mei
Sokka: You mean that creepy lifeless girl that shoots dart from her hand??
Zuko: Yea
Sokka: Why didn't she join you?
Zuko: This is my problem, so I don't wanna let her get involved
Sokka: Well..my girlfriend turned into a moon
Zuko:..................................tough luck kid