Well that is just 1 reason. The most imp reason would be breaking of trust .Why didn't he go to him in the first place confessing everything. Wasn't he who convinced pia to run away. He just proved that "beta aur bete jaisa" are 2 different things.
He seems to know how to behave with his family despite being ill-treated. Then why didn't he go upto Jai and say " hey listen we love each other. Yes Pia accepted your proposal, but she's realised her folly & wants to back out"
And to top it all, he still doesn't get the reason for jai's anger. How dumb can he be.