Mauli is strong and Nandini is weak. Mauli never needed Kunal's help, she is a self sufficient woman who knows how to hide her vulnerabilities, she can't cry in front of people and Nandini needs help, her ability to cry at drop of the hat is her biggest tool.
An example of this is when Mauli got to know about KuNan relationship and Nandini thought she'll commit suicide, Kunal says that Mauli won't do it, she is a strong woman but when Nandini goes missing Kunal says - "Agar Nandini ko kuch ho gaya to uski jimmedar tum hogi"
Mauli can't cry, She can't show her pain, none is there to support her, she has this wall around her and she can't break it, none is even trying to get to the other side and understand what she is going through, everyone from Kunal's family to Kunal is expecting her to be the strong woman, they want her to be the most responsible person at her most vulnerable state, Kunal has placed responsibility of his whole family on Mauli and now he placed responsibility of Nandini's well being on Mauli. She is crying there but everyone is immune to her pain because it's Mauli, a girl who smiles in her sleep, a woman who knows what's best for her, a woman so responsible that mistakes are not part of her life.