What exactly is "hapiness"? 

One the one hand it's not something you can find that easily, since not all people find something that makes them happy.

But on the other hand, it's the simpliest thing in the world, isn't it?

Just take a good look around and you'll see - happiness can be found everywhere by everyone.
You can become the happiest person in the world just because of the smallest thing like a smile of a child or a ray of a sunshine.

Main OST for the story: "Spring Love" by Wendy & Eric Nam.