That dialogue reflects Sahir's desperate need to protect Arzoo, her identity, shield who she is from the world!
Many of us had said that Sahir did not want arzoo to become like him and hence to protect her he did all that he did because he loved her!
Arzoo is a reflection of who he once was, I am pretty sure of that! "Tumhari achai, Tumhari sachai, Tumhari himmat, tumhey duniya mey sab se alag banati hai, sab se uppar" Once that is how Sahir himself was too, he had a lot of himmat, sachai .. despite being an orphan he did not give up on life ... he was a poet, an artist ... a romantic ... he had no one by his side to protect him, so when he was left heart broken by zeenat, in return of all his love, all he got was dhoka ... his world shattered and with it he shattered too!
Arzoo too could have met similar fate or still can, that is something Sahir can't let happen, what life did to him, he can't let life do that to her! There has always been and there still is that desperate need to shelter arzoo from the mean world ... the world that will turn her into another him .. so he will go to whatever extent to be that shield that preserves arzoo just the way she is! His arzoo!