Hearts are earned for customer and cat interactions! You can get bonus hearts by pairing cats with customers of a similar personality. The more hearts you collect, the more baskets you receive!
Cats must also earn enough hearts before they are experienced enough to be Levelled-up (Nurtured). While you may have enough overall hearts, you may not yet have enough hearts earned with a specific cat to level it up. Try pairing this cat more frequently so you can get that kitty levelled!