If purab was irresponsible then he could have just left without even saying anything to jassi. He did not say anything as he was breaking the relation with her and probably would never see her again. Why would he had to care for her and write a note...even though it was not the right note as we all felt...he could have just gone...
He really cared for her deep down and he did not want jassi to come back to him just because she cares for her family and him. He need to break this realtionship in such a way, that she would be sad but not as much. She will then truthfully (to her heart) move towards armaan.
Purab tried his best to show the colors of armaan but jassi was blind folded. If she had heard him then she would have been in less pain and happier with purab. She choose what came to her. She choose armaan over purab at the party and she choose to support armaan with all the actions that were unfair and did all the things unfair to purab. How long should a gentleman be gentle with the lady...