For example, "wife" is "wives" in plural, as is knife, strife, etc. What's the reason and/or etymology behind this?
This is a throwback to Old English. The "f" would be pronounced like a "v" if it was between two vowels.
For example, the word for heaven was heofon and would have been pronounced something like "hayovon" (Sorry, no IPA).
Here's some information from an article on English plurals:
(Adding screen snap here because the markdown here doesn't do tables easily.)