WHY Romeo and Juliet is all time favorite love story? WHY romeo and juliet are famous inspite of its sad ending?

I would say
that one reason it is famous is definitely BECAUSE OF the sad ending.  People love , and the
idea of two people so young and so in love dying after one night together grabs attention. 
Another reason is that popularity is a self-perpetuating thing, and people love it because
people love it.