Why do we say "A man's got to do what a man's got to do"?

Meaning:

One must do what is necessary to achieve your aims, whatever the consequences.

Background:

According to America's Popular Proverbs and Sayings edited by Gregory Titelman (1996), this not particularly profound expression is attributed to a line spoken by John Wayne in the 1939 western Stagecoach. However, in John Steinbeck's The Grapes of Wrath (1939) Jim Casey expresses a very similar thought:

Casey said quickly, "I know this - a man got to do what he got to do. I can't tell you. I don't think they's luck or bad luck. On'y one thing in this worl' I'm sure of, an' that's I'm sure nobody got a right to mess with a fella's life. He got to do it all hisself. Help him, maybe, but not tell him what to do."

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