How did Macbeth become a loyal servant to King Duncan?

Macbeth is renowned as a brave and valiant warrior. He shows his immense courage, as well as his loyalty to King Duncan, on the field of battle. As the play opens, Duncan's enemies have just been comprehensively routed, with Macbeth more responsible than anyone else for Duncan's decisive military triumph. Ironically, Duncan rewards Macbeth for his loyalty by making him Thane of Cawdor (the previous holder of that title has been executed as a traitor).
Yet Macbeth in due course will stoop to much greater depths of treachery and deceit than the previous Thane. His actions are made all the more shocking by the fact that he starts off as such an unfailingly loyal and honorable servant of his king. Macbeth's the very last person that anyone would suspect of having designs on Duncan's throne. After all, his loyalty has paid off; he's been rewarded by the title of Thane of Cawdor. There's simply no need for him to get any ideas about stabbing Duncan in the back. That he subsequently does betray Duncan serves to develop the important themes of loyalty and betrayal, as well as driving the plot forward.