We are blaming Armaan & Jassi for not believing each other. But the question is was the trust word there ever between them ???
Armaan created a long & painful misunderstanding between Purab and Jassi coz he was always scared of loosing his company.He always used Jassi's emotions for running his company since he is handicapped without Jassi.
Even when Jassi's engagement was fixed with Purab he tried his best to attract Jassi towards him, including the meeting on her party night which could have waited.
It is not only the CJ scene that shook his belief but i guess Armaan never trusted Jassi as much as she trusted him. No matter how much his mouth says so .