Kyun Chalti Hay Pawan?
Because of evaporation

Kyun Ghoomey Hay Gagan?
Becuase of earth's rotation

Kyun Machalta Hay mann?
Becuase of disorder in digestion

Kyun Gum Hay Har Disha?
Because you have poor sense of direction

Kyun Hota Hay Nasha?
Because of drug addiction

Kyun Aati Hay Bahaar?
Because of change in season

Kyun Lut'ta Hay Qarar?
Because of taking tension

Kyun Hota Hay Piyar?
Because of opposite attraction
..
..
I've explained you everything
ab mat kehna
.
"Na Tum Jano Na Hum" ;) :D