There are two main schools of thought when it comes to taking street portraits, those who ask for permission prior to taking the shot and those who don't. The latter will always argue that the element of realism is lost if permission is sought, as most willing subjects will pose for the camera and the scene will be contrived.
The former will decline from ambushing unsuspecting members of the public and will argue that the permission approach will make them far less likely to get sued, arrested or even thumped. They may even refer to sources on Youtube like this one to illustrate their point.
For me the jury is still out. On this occasion I opted for the permission approach and asked this gentleman if I could take his picture. He readily agreed, on the condition that I send him a print of the photograph. Like the picture or loathe it, I will take great pleasure in keeping my side of the bargain.