I am !!

I think I am what you think I am.

When you thought I am a friend, I was.
When you thought I am happy, I became.
When you said I am no good, I worsen.
When you said I am the best, I was better.

I was quiet when you wanted me to.
I was there when you needed a support.
I left when you thought I would.
I came back just when you thought I should.