Historical reason

A word is retained for a name though the meaning has changed because the referent has changed. (E.g. pen )

Increased scientific knowledge and discovery is also an important factor that underlies the change of word meaning. (E.g. atom)

 

Class reason

Language records the speech and attitude of different social classes, so different social varieties of language have come into being.

The attitude of classes has made inroads into lexical meaning, particularly in the case of elevation and degradation.

Quite a number of words that denote different sorts of working people or their occupations have deteriorated and now have taken on a pejorative sense. 

E.g. Churl, hussy, wench, villain which were originally neutral in color have been down-graded as “ill-manned or bad people”.

Psychological reason

The associated transfer of meaning and euphemistic use of words, etc. is often due to psychological factors.

People change word meaning owing to various psychological motives: love, respect, courtesy, suspicious, pessimism, sarcasm, irony, contempt, hatred, etc.

The fact that some low, humble and despised occupations often take on more appealing names is all due to psychological reasons.

Religious influence is another kind of psychological need.

Linguistic factors

The internal factors within the language system.

One type of such change occurs when a phrase is shortened to one word which retains the meaning of the whole.

The influx of borrowings has caused some words to change in meaning.

The competition of native words eventually yields the semantic division.

The change of meaning is brought about by analogy.