Sometimes to "friend" someone means to follow their lj because you like what they post, on their lj or in communities, sometimes it includes mutual interest (mutual "friending") without personal attachment, sometimes actual friendship is there to begin with or may grow, to hopefully a similar level of comfort for both parties. Mutuality is delightful when it happens, but should never be expected.
"Friending" someone back usually means you allow them to see some or all of your locked posts (unless you employ filters), but it is NOT essential.#
It's highly unusual to send messages to ask someone to defriend you. When it happens the person asked is likely to feel upset and wonder what they have done to get such a direct approach, because essentially what you have done is yell "stop following me!" at them. Asking someone to defriend purely because they haven't commented in a little while, or commented somewhere else instead of on your post is rather rude, actually.
There are less personalised ways to tailor your reading list (your friends list, of people whose journals YOU want to read). If you have mutually friended people who friend you, and you don't want to see them on your friends list any more, then you can filter your reading list. Alternatively, once in a while it is fine to post and say you are culling your flist to those you interact with regularly. This has no effect on folks who simply like to see your unlocked posts, although they may decide to remove you from their friendslist too. That's up to them.
# I am not going too far down the rabbit hole of locked posts and filtering, because I'm old school enough to mostly post unlocked. If people see something to comment on they generally will. If they can't think of something to say they won't and that's fine too.