Zuko was looking at you with a pang of jealousy, you wondered why, well, you were his platonic love and at the same time his best friend. He didn't want to have these feelings since he wanted to focus on capturing the avatar.

Their jealousy arose when they boarded a place and a high-status boy from that town wanted to marry you.

"You belong to me" He declared out of nowhere since you were thinking about whether to accept or not.