Purab had come to Mumbai to buy a fashion house and meet Jassi. He also falls in love with her. Aryan goes behind everyone's back and tries to sell Gulmohar to Purab. Purab is not really interested until he finds out that Jassi works for Gulmohar. So, he decides to take over the company and gift it to Jassi. Aryan goes about collecting everyone's shares except Raj and Armaan's as he wants to usurp the chairman's position once Purab takes over the company.
However, Armaan and Raj find out about Aryan's plan and also find out that Jassi is one of Purab's closest friends. They basically depict him in a very bad light to Jassi and say that "The Takeover King" was using her etc, etc. This is how the misunderstanding starts. In fact, Purab had asked Jassi about Gulmohar, Aryan and even the shares a few times, but she always avoided that topic as she was planning to join IFH at that time.
So, she gets mad at Purab and rejoins Gulmohar thinking that her Armaan Sir is a devta. Purab rejects Aryan's offer as he didn't want to hurt Jassi. He also writes that letter which Jassi reads NOW after all the mess explaining himself to Jassi.
I hope I've covered everything.