After this experience, I found two other colleague/friends who enjoys goofing around the karaoke scene just as much as I do.
 We went again last weekend.
For the past few days, I kept singing the same songs that we sang that night repeatedly.
People around me were wondering why am I in such good mood.

I realised something.
 I'm singing not because I'm in a good mood.
I'm in a good mood because I'm singing.

Years ago, I learnt about the behaviour + emotions + thoughts correlation in CBT.

But it is only now that I understand.