Simply because they are not forced like cheapo PavJaz romance (yucks) or Nikky Jaan showmance (yucks).
When both of them were consoling Kavity yday, it was damn good scene. Rahul and Rubina both are cunning and mature in their own way. Their rivalry is something that developed naturally and was never forced where one just wanted to fight with another, and that's what makes their scenes so popular.