Irrespective of when and how Rahul fell in love with Anjali; Anjali has been in love with Rahul from the very beginning. While she loved Aman and chose to marry him, she never really got over Rahul. Now that Rahul is back in her life - she has a choice - to be with the person she has always dreamed of or settle for Aman. If she chooses to marry Aman - she will always wonder "what if" and Rahul will continue to be the one that got away.
Her mother Reema Lagoo sees that turmoil and tells her its ok not to settle and to pursue her dreams.
Rahul never appreciated you back in the day and so his love is shallow is the sensible practical advice. But when it comes to human emotions, you can pretty much throw logic out the door.
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