Hearts are earned for customer and Bunny interactions! You can get bonus hearts by pairing Bunnies with customers of a similar personality. The more hearts you collect, the more baskets you receive!
Bunnies 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 Bunny to level it up. Try pairing this Bunny more frequently so you can level it up!