Like a lot of RPGs with affection systems, I imagine we're supposed to learn the preferences by trial and error. As arbitrary as it may seem, there are some consistent patterns to what party members approve of, though some obviously clash with each other. Gale likes Diplomacy and Conflict Resolution, Lae-zel likes forcefulness and agression, Astarion likes showmanship and confidence, and Shadowheart likes manipulative and aloof behavior. Some of that is readable from their personality and the things you learn about them in conversation.
