Every song has a history to it.

You plug in those earphones and then among the thousand songs, you scroll up and down to chose your song. What goes in the back-end?
The simple reason is that you like the song more than the other songs in your I-Pod, or particularly you relate to that particular song more than any other song in the playlist.
Why does this happen?
I guess because all the songs have a history attached to it.
Its so often that you listen to a particular song and you are lost. In those moments of your past, those cherished ones, the ones which brings smile on your face, or tears in your eyes. But certainly those moments which are etched in your mind and that particular song helps you in keeping those memories afresh.

A yaaron dosti badi,Puraani Jeans, Pal-yaad aaeyenge ye Pal, would take you back to your college days. Reminds you of those moments spent in the canteen with your friends pulling each others leg, passing comments on the teacher who is also eating somewhere there.Talking about journal submissions or the latest couple on the block.

Whenever you listen to Aye mere watan ke logon, it would take you to your school days where you used to reach early for the flag hoisting and you waited eagerly for the sweets to be distributed only to be overjoyed with the fact that there were no classes on the independence/republic day.

Whenever you listen to Phoolon ka taaron ka, it would remind you of those Rakshabandhan days when you used to ask your parents for money to be given to the sister and those Rs. 50 made your sister the richest girl in the world who thought of buying the best dolls from that money she had just received.

And there are so many songs like that

That is why I say there is history attached to each song. Each song from your past reminds of you of those old good days, to which you can only say, boy those were the best days of my life.