Henry Fonda one time said that every time he had a job, he thought it was gonna' be the last one. And, if you got any sense, you gotta' think that because, you know when somebody's gonna do a dip, some of 'em go pretty far down.
For an actor, you're rejected eight or ten times a day. All you've got to sell is yourself. You're not selling products, they're not turning down a car, they're turning you down. Most people can't handle that. Most people are essentially not set up that way.