I got a heart for Ste either during or in the scenes leading up to the Country Road Expansion Quest where you get a road built that connects Ste's place to the rest of the map w/o tunnel use... While I do not know what my like was with him prior to the quest, it is currently at 58 and I raised it through quests, working on the field with him, and playing with his kids. If I had to guess, the quest is unlocked once you reach a certain affinity.