Looking into the minds of criminal is always quite an interesting task because it provides us with some real psychological answers into why and how people act in manners that go against the conventions of society.

There is no specific reason that causes someone to be a con artist. Their lifestyle, financial situation and the people they associate with all play a role.
When we are talking about con artists, we need to differentiate between them and people who steal for other reasons.
Con artists:
These people are also different from other robbers because they are essentially putting on a show with what they are doing.
Another reason that people steal from others is because they feel that it is a "necessity." They think that in order to get any money, they must take it from other individuals. Therefore, they become con artists in order to get what they want.
When we think about situations like that of Oliver Twist, we almost feel bad for some of these people. Poor Oliver really had no choice but to act as a con artist, and if he did not do as he was told, who knows what type of punishment he would have received?
Sometimes the environment or culture creates a tradition.
Think about the scenario of Oliver Twist again. While he was acting out of necessity, he fell into a sort of "tradition" as well. All of the boys living where he was were engaged in pick pocketing and scamming activities, so he too had to join in on it. It sometimes makes it easier to scam other people out of their money when you have the support of a group to do so.
Let's bring that example to a modern day situation. If you are born into a family of con artists, chances are that you are going to be a con artist too, for at least some portion of your life. Maybe you are part of a sub culture that engages in these con artist activities. Being a con artist will be pretty much ingrained into your lifestyle and infused into the way in which you are brought up.
You will be taught how to act as a con artist from early on, but in the future you might decide to follow a different path in life.