From Psychology Today blogs, Paul J. Zak discusses the anatomy of con job:
The key to a con is not that you trust the conman, but that he shows he trusts you. Conmen ply their trade by appearing fragile or needing help, by seeming vulnerable. Because of THOMAS, the human brain makes us feel good when we help others--this is the basis for attachment to family and friends and cooperation with strangers. "I need your help" is a potent stimulus for action.
Also see this video of Michael Shermer running the "pigeon drop" on the streets of Westwood, California: