What is happiness?

What is happiness? In my point of view people should be optimism to everything even when they are in trouble.
I have heard a story that a man lived in northern frontier of China lost his horse one day. Everyone comforted to him, but the man said how do you know it would not be a good thing that I lost my horse. Unbelievably, the lost horse came back after several days, and it even brought a mare with him. People all said to the man you are so lucky, however the man himself said how do you know it wouldn't be a bad thing. One of the man's son loved to ride, but one day when he rode he broke his leg. Therefore, bad things and good things both lie on your mind, if you think the thing is good then it is good to you, and if you think the thing is bad it will be bad.
At the same time there was a half bottle of water, one people saw it, and he said" o~ there is just a half bottle of water", while another person saw it, he said " o~ thank god! I still have a half bottle of water". For this reason, optimistic people will find happiness from their life, but pessimistic people think everything is bad.