Jassi, had very honestly told Purab about Armaan and she beleived she had got over him and she was marrying Purab and if you remember she had got Armaan out of her thoughts. She was doing the right thing.
It was Purab's ex girlfriend who ruined everything on the evening of the party. What did Purab do? A guy who was most welcomed in Jassi's family never made an appearance but deserted Jassi and Bebe his best friend without saying a word.
Purab really is shown to be a screwed up character!
Armaan with all his faults is a very week character and Jassi knows all about him. Jassi is very strong and time will tell if she is capable of accepting him as he is.