What is the difference, if any, between 开心 and 快乐 Dictionaries will give "Happy" for both.

A Mandarin song has the title 你开心所以我快乐

https://youtu.be/Sb1ojccxvyw

Question therefore is whether there is any signifying difference in usage?

That is, which word / phrase fits better in a certain context or situation.
开心 can be either joyful, delighted or the mood of happiness.   

快乐 is the mood of happiness.  The dictionary defines it as:


  形容称心如意的心情


So, we say 我今天玩得很开心，not 我今天玩得很快乐.  

But they can be synonyms as in 和他在一起很开心/快乐. In this case, both denote the sense of the mood of happiness.