Build 15 houses, cottages or manors. 2 Keflings per gives you 30 in total.
If you are having trouble finding hearts, sometimes they are hidden under resources. So be sure to chop all the trees down and mine all the rocks/gems that ARE NOT infinite. Also try doing quests for the Mayor or Lord. You will find additional hearts and items that way. (Compliments of Ry Fryy and Stoodbridge)