A Happy "Unfortunate" Tree

Someone once told me that being happy is my nature. However, I have been carrying around a heavy heart since I was a small child. I met the person again when I just turned 22. Thinking about all the hardships I went through, both the inner and external restrictions, I could not see what is there to be happy about. He said, one day you will understand. I know he means for me to change my perspective. But how?