Good topic Tina!
It's definitely a difficult one. After watching the episodes for a second time, it seemed pretty impractical for Jassi to be fantasizing (even before Armaan said ILU to her) that Armaan would love her. I definitely don't mean because of her looks, but because Armaan was an engaged man and already entangled in another relationship with Mallika whether he liked it or not.
I remember the first time I was watching JJKN initially, in the days that Jassi would dream about being with her Armaan sir, I never in my wildest dreams thought that one day the two would actually get together. (And at that time, with Purab in the picture.. I had really thought that she was going to end up with Purab because there would be less conflict that way. đ ) So you can imagine my bewildered reaction when Armaan said ILU to Jassi on his engagement day.
But yes, I do think that it was Armaan's fault for playing with the emotions of both Jassi and Mallika at the same time. And although we can say that Jassi should have realized that Mallika & Armaan were already engaged and that she was fantasizing about an engaged man, as someone said earlier, love makes you go crazy even to the point of defying all logic and discerning capacity between right and wrong.