Riddhima loved Armaan more than life itself. When he left she was left without her spirit - lifeless. Then Sid appeared like a storm into her life and shook it upside down. Fighting against him, fighting with him, being with him looking for her lost life, losing her purpose in life all over again, marrying him, ending her life and then the final breath of life when he showed her what she had become and how disappointed he was because of his expectation from her, all of this slowly brought her back to life. And bit by bit every moment with him after that has brought her closer to life. And without realizing it she changed from a woman who wanted to die to a woman who wanted to live. She chose to embrace the life offered to her, she began to love it.
She was deeply shaken when faced again with her past. She cried at the injustice of life which had put her in a situation where she had to choose. Despite all the warring feelings in her heart and mind, she realized that she had embraced her future with Sid whole heartedly, and now there was no turning back as by now her feelings ran too deep.
However, she was shackled by the guilt she felt for having moved on while Armaan was still where she had left him. When Armaan asked her to forget about him and to work on her marriage, she was finally free of what held her back. Now she could move on without any baggage, from then on she could feel for her husband without any guilt. And that is what she did. She thought Armaan had developed some closure too, and this was a relief.- this is why she didn't see Armaan's pain. Because finally he had set her free and she could revel in the love she felt for her husband.
But the diary fiasco revealed to her that Armaan still loved her as deeply as ever. His sacrifice and pain became painfully apparent to her. She had managed to move on without him. But he hadn't. He only thinks about her happiness, even at a great personal cost. She is reminded again at what a big heart he has. And this causes her immense pain and guilt.
When she asks why are you so good, why must you do things that make me think of you, I see this to mean that she had managed to put thoughts of him behind her and be released from her guilt, but once again she is faced with Armaan and his love for her, and with it her guilt. His causing himself pain for her happiness multiplies her guilt. When she says why can't I be like you, I see this as her guilt at why she was able to move on and be selfish while he couldn't do the same. When she asks him why do you love me so much, its because of her guilt of why she couldn't love him as much as he always loves her. For me the root cause of her lamentations is her guilt at causing him such pain and falling in love again.
For me, this AR scene was beautiful. It may have been Riddhima finally confessing that she never could love this awesome man with such a big heart as much as he always loved her, and the guilt weighs her down.
This confession was necessary for her to be released from her guilt. This showdown was necessary for both to get some real closure.
Nisha