I think more than actually falling in love with the person, she fell for Raj because he was her childhood friend. Simi had once told Preet that when she grows up, she too would fall in love. She even told her to marry Raj later! I think that was playing on Preet's mind big time.
No guy would have ever proposed to Preet, because nobody knew she was a girl. She wouldn't have even experienced having a crush on someone to be able to distinguish between infatuation and love. So the first guy who came along and swept her off her feet just happened to be Raj Bhajwa who was also her childhood friend and whom her Simi didi suggested she get married to.
It's also possible that she married him out of fear - to avoid marrying a girl AND because she was scared she would never ever get married if she lets the one in hand go!!
My opinion is that Preet never really loved Raj for the person he is. She's only fooling herself into believing that she loves him!