They've said or shown us all of what I've said below.
They were friends in their childhood due to their fathers' friendship. They played together but were being brought up very differently. He was already developing a deep sense of responsibility towards his family and supposed position in society. So he always needed to be proper and stay within his lines, not show too much emotion. She didn't have any of these constraints. She could just be a child - a slightly cheeky one at that. Maybe her being cheeky irritated him, maybe it made him realise what he was missing out on. Ether way his way of dealing with it was to stop talking to her completely. Cut her out from his life. Did she miss him? Yes she probably did but she learnt to deal with it because she was surrounded by her family's love and all her friends. Anyway if he could stop talking to her so easily just because she was a mischievous and wanted to have fun probably meant he was not her kind of guy anyway.
Fast forward many years and anurag is back from London. He is the bright topper, polite, kind and respectful to everyone. She has built a world for herself - she is busy helping her family, having fun with her friends and studying (even if it's stuff she doesn't like but thinks she needs to because it will help her family). So she keeps a polite distance but thinks highly of him. He is intelligent and him joining her college has been good for it. She knows not to get too friendly - he doesn't like someone like her and will probably stop talking to her if she does something he thinks is wrong. Also it's been years of being treated like a second class citizen by kaki-ma and Nivi while anurag was away - she knows her place in that family and has even stopped visiting or going to their house. And him - he notices her. He knows when she comes into a room, when she's around. Even when he can't see her. He's just out of a relationship but can't help thinking of how beautiful she is. But he's not interested in bringing her into his tiny circle of friends- he knows from his childhood how different they are. He's even more the gentleman now, determined to make a respectful name in society and not be known as just Moloy Basus son living a rich mans life. He knows what his role in life is and a friendship with a crazy girl has no place in that. Anyway if there is anything his past relationship taught him its that there is no such thing as love.
We all know where we are today in the Anurag-Prerna relationship. If there is one thing neither will want is to lose the other. Anurag will always be inclined to do what he thinks is the right thing for the son of Mohini and Moloy Basu. It's what he's done from childhood. The only difference is that rather than cutting prerna out from his life for being different, he is desperate to keep her in his life. Prerna has always wanted Anurag in her life after spending years without his friendship. She is like her father - loyal and willing to do anything for a friend even at the risk of her own success or happiness. He is the one who couldn't handle her. But now she knows that what she feels for him is not friendship, it's love and that love only makes her sense of responsibility, loyalty and protectiveness ten times stronger.
This is not a couple that really fight for their relationship or know how to, this is a couple that fight for each other. Someone somewhere needs to show them that actually today, the people they are, they are nothing without the other.