What is Happiness?

I always wondered what is happiness? I see a few people being happy with just a small positive happening in their lives verses, I see people not happy with a lot of big achievements/ gains in their lives. So that concludes me that happiness is nothing to do with gains and failures . I have personally witnessed a few people in my life who find happiness in very tiny things inspite of all harshdeep around them. They trained themselves to find happiness in everything they do or come across. 

So I came of this  based on my experience and observation :

" Happiness is a state of mind, one can achieve by training, its nothing to do with circumstances around you"

Would like to know your insight on this, please do comment.