Man the machine--man the impersonal engine. Whatsoever a man is, is due to his MAKE, and to the INFLUENCES brought to bear upon it by his heredities, his habitat, his associations. He is moved, directed, COMMANDED, by EXTERIOR influences--SOLELY. (What Is Man?, Mark Twain, http://underthesun.cc/Classics/Twain/whatman/Whatisman.htm)
There is some scientific basis for this claim. Studies have shown that both a person's genetic structure and the circumstances to which he or she is subjected have bearing on how a person thinks, feels and acts. Considering this, the actions of the character Macbeth must be evaluated by his personal motivations and the external causes that may have led to them. It is established from the very beginning that Macbeth is ambitious. There can be no doubt about this. A certain level of courage accompanies his ambition as well. As a noble he is an active one, fighting against the rebel hordes and Norwegians in defense of his king, no doubt for the purpose of gaining notoriety and other rewards. This is further illustrated by his gracious acceptance of credit for his deeds. He is a political figure in the highest sense, and show ambition in this way. However, there is no sign of him altering his course of loyal nobleman until outside influences begin to intercede. The people with greatest impact on Macbeth are the witches, his wife and Lady, and King Duncan of Scotland. The witches introduce the idea, King Duncan gives personal motive, and Lady Macbeth helps along the way.
The least influential party in all of this is King Duncan. The conflict between these two is purely circumstantial, but clear enough. Macbeth is, as stated, an ambitious man. The King represents the highest position of power that Macbeth can hope to achieve. The King is also a father figure, patronizing to his subjects and expectant of total servitude.
When King Duncan thanks Macbeth for his heroic service in battle, Macbeth replies that "Your highness' part / Is to receive our duties; and our duties / Are to your throne and state children and servants" (1.4.23-25). Macbeth's metaphor expresses a common idea of the time: A King cares for his people as a father cares for his children; and the people are supposed to act like obedient children. (http://clicknotes.com/macbeth/Babies.html)
In this way, Duncan's very position is likely to grate against Macbeth. His actions do nothing to endear the King to Macbeth either. While rather obnoxious and rude to Macbeth, as well as snubbing him for a shot at the kingship, His Majesty is mostly unwitting to anything that is going on. Duncan's main influence is directly after Macbeth is honored for bravery and courage in battle, fighting for Duncan against a rebel lord. Macbeth is busily basking in his own glory and soaking up credit when Duncan basically steals his spotlight from right over his head, proclaiming Malcolm, Duncan's son, as the heir-apparent.
"My plenteous joys, wanton in fullness, seek to hide themselves in drops of sorrow. (In reference to the nobility of Macbeth. He switches gears rather quickly.) Sons, kinsmen, thanes, and you whose places are the nearest, know we will establish our state upon our eldest, Malcolm, whom we name hereafter the Prince of Cumberland;" (Act 1, Scene 4, Lines 39-45)
This action also belittles Macbeth's achievement, since the procession of the throne is not necessarily dictated by bloodlines. Duncan is basically announcing that Macbeth, while noble, is inferior to his wonderful son Malcolm, and deserves a nice spot in the sun even though his actions were less. This is where Duncan provokes Macbeth to hate him and also points out what Macbeth must do...