Why do words change meaning over time?
Because meaning is negotiated in use rather than fixed by definition, and the changes follow recognisable patterns — which is why the process looks like decay from inside and like ordinary language change from outside.
The main patterns:
Broadening, where a specific term widens. Dog once meant a particular breed; holiday was a holy day; arrive meant to come to shore.
Narrowing, the reverse. Meat meant food generally, deer meant any animal, girl meant a young person of either sex.
Amelioration, where a word improves. Nice meant ignorant, then fastidious, then pleasant — one of the most travelled words in English.
Pejoration, where it worsens. A striking number of words for women have followed this path from neutral or positive origins, which linguists treat as evidence about the culture rather than about the words.
Metaphorical extension, which is the largest source — physical terms extended to abstractions. Grasp an idea, a warm welcome, a sharp remark.
Semantic bleaching, where intensifiers lose force through overuse. Very meant truly, terribly and awfully once meant what they say, and literally is simply the current example of a process that has already consumed several words.
Why it accelerates or slows. Contact with other languages, technology needing vocabulary, social change, and generational renewal all push it; writing, standardisation and prescriptive education slow it.
Why it provokes such strong reactions. Objections cluster on changes happening now, while identical completed changes go unremarked — nobody objects to nice meaning pleasant. The linguistic position is that there is no point at which a language was correct and from which it has since declined, and complaints about decay have been recorded continuously for as long as writing about language has existed.
Where the criticism has force: genuine ambiguity in contexts requiring precision — legal, medical and technical — which is why those fields define their terms explicitly rather than relying on ordinary usage.