Tanya has given her entire life tomake Karan Happy and Karan is indebt to Tanya. Karan respects and cares for Tanya but doesn't love her.
There is a very popular movie called Mein Tulsi tere angaan ki, where Asha parkeh is the other women and Nutan is the wife of Vijay Anand. Vijay loved Asha and never had the same feelings for Nutan.
Though we all want Nandani and Karan to unite, but some love aren't meant to betogether phycally.