We usually do not appreciate things which we easily get in life. Dev realized his love for Sonakshi, went to propose her and she accept it instantly so Dev was unaware of the pain of losing. He thought that he is doing a sacrifice for his mom but little did he understand what it actually feels after breaking up.
On the other side is Sonakshi who got her love after almost losing it. She went through a heartbreak even before their relationship started and so she valued the relationship more and went far and beyond to nurture it.
Before it was Sonakshi who was going through the pain and anxiety seeing Natasha in Dev's life and now it is Dev who is feeling the same seeing Rithik in Sonakshi's life.