Happiness can be obtained by clearing Rubble and building more Decorations on your Farm. You can unlock different Happiness Milestone Rewards with the Happiness Points that you have collected.
Tap on the Daisy icon in the upper left corner to view the Happiness meter and the Happiness Milestones.
Related posts:
FarmVille 3: What are Decoration Coins and how do I earn more?
FarmVille 3: What is the Order Board?
FarmVille 3: How do I make room on my farm?
FarmVille 3: How do I see my Followers / Likers?
FarmVille 3: What is the Sky Race?
FarmVille 3: How do I plant and harvest crops?
FarmVille 3: How do I use the Building and Decor Shops?
FarmVille 3: How do I change the appearance of my buildings?
FarmVille 3: How do I edit Buildings and Objects?
FarmVille 3: How do I use the Barn Storage?