Rachna and Mayank are cousin brother and sister and story started when Gunjan (cousin sister of Rachna from her mother's side) comes to stay with their family after her mother's death. Gunjan was shown to be flamboyant girl and Rachna was shown to be timid. Both characters had their own set of fans, but Gunjan's character dominated screen space.
Gunjan fell in love with Rachna's cousin brother Mayank (now she is married to him) and Rachna had a crush with her teacher (Rajiv) which did not materialise (But that story being cute and innocent continues to have its own set of followers). Next; Rachna's marriage got fixed with a rich spoilt brat Vihan, who broke up with her on their engagement day but fell in love with her later. This hate-love story also succeeded to attract audience, as Vihan had the bad boy charm and Rachna was the girl next door. But their story was rushed (as Rachna's character used to get less screen space). This made the actor to quit, just like Rachna's previous lead Rajiv. (Before he left, we had an episode where Rachna breaks up with him citing her family and self respect. He moves to Canada. So the option of his re-entry is open. But most probably he is not entering SSLK again).
Now they have brought in a new lead again for Rachna as the neighbour Kabir (who happens to be Piyush). Rachna seems to look more matured than what her character used to be in the beginning of the show and Kabir is getting more screen space. So we are hoping, this pair is here to stay.