Disclaimer: I don't own Avatar The Last Airbender or its characters
Their relationship was like water.
Going with the flow, it was natural and easy. They always had things to talk about - one topic moulding into the other with occasional bursts of laughter. Utter happiness at having a friend; one of many friends. At times of despair, each other's company was essential for them to be reminded at how much love there is in their lives - not only coming from each other, but from everyone else each of them had found important.
It was soothing. When around each other, they had found themselves comforted and calmed down. They were each other's shoulder to cry on, especially because there was full understanding between them - a similar backstory, similar worries, similar way to express them (they could never blame each other for occasional bursts of anger).
Their relationship would always be different, like a calm lake and a storming ocean. That's what had kept in interesting - contrast. Cool and soothing like ice; potentially hurtful to both of them like boiling water; unstable like steam.
Water is Katara. Katara controls water. Katara had always controlled their relationship. Katara could take it from one extreme to another - Zuko could only help her make it less stable when she would be especially willing. At the end, it was Katara's choice not to start anything. Not Zuko's. Because their relationship was like water.