Because:
1) astha saves shlok from sticks and they roll.
2) shlok saves astha in jungle from dog and after he comes out of pit they roll.
3) shlok saves astha from fire and they roll.
4) shlok saves astha from horse and they roll.
5) shlok saves astha from drowning in jungle and pushes her out making her roll.
And seeing all these our eyes roll
That's why is pyaar ka naam rolling pyaar...I hereby declare ashlok name as RJ aka rolling Jodi of the year...who all are game to use the new name??