You need to buy blooming heart. You can byu them with Life Crystal. one for 150 LC and 4 for 250 if I'm not wrong. And I think there is also other way to buy them with another currency but I don't remember what currency exaclty.
also, it's not that important but the blooming heat don't reset the affection counter. It unlock the bloomed affection counter. You still have the previous affection bonus and just add the bloomed affection bonus to it.