Well people scam because they don't have enough or not contented with what they have in other word being criminal minded could be another major factor that may lead people to scam other people.
Mostly Scammers are person who gain trust from their victim that has bad intentions and motives. They are skillful that can fool everyone who do not have knowledge on what scammer do. Sometimes those person are soft spoken and very good in sales talking where many can buy because of their words. Person that easily disappeared after getting what they want from you.