I hate it when nurses do the first person plural thing to patients, like when my husband was in hospital. "Now, dear, we want to eat some of the nice soup, don't we?"
"No, we don't. You might, but I really really don't want to touch that muck." (Genuine response from my husband!)

But apparently, from a psychological point of view some people use the third person not because they are pompous or vain, but because they have a low sense of their own identity. That's why when you read the Bible, most of the books allegedly written by Moses, or David, or Samuel, or whomever, are in the third person. They didn't want to big themselves up if the hero of the story was God.

That's one of the clues as to which later books are forgeries... for example, the Book of Mormon literally starts of with a flurry of first person singular (I, myself, me...) which is an anachronism for the time it was supposed to have been written.

Some early commentators on the new testament used to accuse the New Testament letter writers of being vain because they stated so clearly who they were, whereas the gospel authors kept themselves out of the main frame by writing in the third person (allegedly.) Other ancient written traditions have the same focus.

So, in that tradition, if I said, "Late at night many people could not sleep, and so wrote messages on the internet. Mgran waffled on so much that people finally fell asleep", I'd be considered to be being polite. Whereas if I wrote "Last night people couldn't sleep, so I wrote messages to help them doze off" I'd be considered to be a screaming egocentric.

Like most things, it's a matter of the culture we live in.

I hear it can be a sign of an identity disorder. But I do know (mgran knows) that it's not always to do with vanity, or humility... sometimes it's just a style of speech. Nothing wrong with that.