The Landing

Jackson was the 100th delinquent on the drop-ship spite his age. He committed a crime in Medical and Abby convinced Kane and Jaha to spare his life and send him down with the rest of the 99 so they have a doctor down there just in case they get hurt and to keep them in order. Abby's real reason to send him down is so he can keep a eye on Clarke.

On the ground when there was a grounder attack Jackson got attacked but Miller saved his life killing the grounder and Jackson started to spend more time with Miller since he started to fear the grounders. Miller and Jackson end up falling in love.

This is how they fell in love.