u gals all had great points......however, personally I am leaning towards the direction naina has mentioned.....
see the way I see it, ever since Jassi has agreed to marry purab, she has treated him rather carelessly........lately she is seeing him in a new light and not just as an extention of herself......where before she was content at the way things were, now she has begun to question what Purab sees in her .....more to the point, the intoduction of Meena has made her realise that there is no shortage of intelligent gorgeous women in Purab's life (though how she could have thought otherwise especially with Ides in Purab's life is beyond me😉)
Also, she is more or less in the same position with Purab as she was with armaan...........she had no scope of a relationship with armaan, so his flirting, she took in stride.....Purab on the other hand is her intended, and she will not accept the same behavior from him...... so its logical that she be more concerned with what goes on between P&M....
Although it can be argued that sheetal too could instigate this self improvement project, I think what makes Meenakshi more threatening to Jassi is that she is not only beautiful but brainy too........so this very thing will promt Jassi to do an inventory of her self and improve on it(hence her looks)........
and the fact that armaan's evident interest previously never complled Jassi into changing her self, speaks pretty highly of her feelings for Purab😉