But why?

I'm listening to the music all the day long — rarely I'm in the such mood.

Damn, it's uncomfortable. Why headphones are tend to be symmetric? To distinguish the left ear from the right one you should have a good look at a tiny label. They should make a little pimple at a dynamic. If you feel the pimple from the left then you got it right.

If I'll be in the same mood tomorrow, I'll stick a piece of tape.