Amelioration, whereby a word takes on favorable connotations and deterioration whereby it takes on pejorative associations, are often telling indications of social change. There is a particularly pregnant category ably defined by C.S. Lewis as 'the moralization of status words' (1960) . . .. By this process terms originally denoting status and class slowly acquired moral connotations, favorable and otherwise, evaluative of the moral conduct commonly attributed to that class. Hence, villein, a medieval serf, and Anglo-Saxon ceorl, still lower in the hierarchy, deteriorated to villain and churlish, while noble and gentle, predictably, rose in moral connotations. In more recent times, the steady amelioration of ambitious and aggressive reveals a change in attitude towards those who seek advancement or 'success' in a highly competitive fashion."
(Geoffrey Hughes, Words in Time: A Social History of the English Vocabulary. Basil Blackwell, 1988)