Katara's Ring of Netorare

The Wind spirit wanted the restoration of Air Nomads, which required the last airbender (also the Avatar) to have as many children as possible. So just having a single wife and 'sweetie' would not produce the rebirth needed.

Which was why the spirit was in disguise as a merchant in front of the Avatar's waterbender girlfriend. Which is why he enchanted the Ring of Neotarare, powerful enough to affect even the Avatar, with a power to attract the woman and maker her want it. Want to wear it always.

"I love this ring, but I can't afford this." said Katara, who had already put on the ring to try it out on her finger.

"I can hagg.. wait, you're the Avatar's girl friend. Much of my family had died in the war, till you and your friends stopped it. Take it as a gift of thanks. My pleasure to give it to you for all your aid to my people. It is a gift and I will not take it back." said the spirit as merchant.

Influenced, Katara agreed. Ring on hand, the Spirit smiled but was laughing on the inside. As the ring would make sure Avatar Aang would take many lovers. Hopefully fathering many children with those lovers.