What is the difference, if any, between 开心 and 快乐

Question

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.

Answer 1

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.

Answer 2

In your case, both 开心 and 快乐 are used as adjectives, which denote “happy”. Translated, the sentence means “I am happy when you are happy.”

However, only 快乐 can also be used as a noun.

“他从做善事当中获得快乐。”

This would mean “he obtains happiness from doing good deeds”. It would sound weird if you substituted 快乐 with 开心, which would then make it “he obtains happy from doing good deeds.”

Answer 3

I think they can be used interchangeably. When it used in songs or poems, rhythm is more important than which word to be used. Sometimes, to avoid using the same word in one sentence, people tend to use a different one too.