Animals are locked by XP level, and happy animals give out XP. To level up your XP, choose an animal and click on the candy wish bubble to select a FREE treat! You decide when you want to come back. When the treat is gone, a red heart will appear over the animal.
Click on it to get XP, but don't be late! Animals with broken hearts (purple heart with a crack in it) will not give full XP. You can also collect your hearts a little early (when they turn pink), however you won't get the full XP amount for that treat if you don't wait the full time.