"Polk was by nature an introvert but out of political necessity forced himself to mingle. He had few genuinely close friends. Still, he was generally well liked. A classic overachiever, he was very ambitious. Biographer Charles C. Sellers attributed his "feverish drive" to "early physical inferiority," "frustrations of his boyish ambitions," and "his mother's stern gospel of duty." To compensate for a lack of brilliance and charisma, he, according to sellers, "drove himself ruthlessly, exploiting the abilities and energies he did possess to an extent that few men can equal." Yet he kept a firm rein on his ambition, never letting it threaten his career."