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Not sure how far the “friends” factor goes really. If we take it literally, friendship is not a reason to merit someone, and in fact can and has in the past lead to unpleasant situations.
The core driver is (and should be) content, although we are all subjective beings, and as such, will favour one sort of content over others (or understand it better). With time, certain profiles will be repeatedly merited by one, not necessarily due to a specific shine on them, but rather because they tend to create posts that are more appealing to them than others. That’s why the more people meriting the better, so as to get a broader range of diverse criteria rolling.
There are also a couple of threads, both on Beginners and Help and Reputation where allegedly good posts that go by unnoticed can be reported, in order to postulate to being merited by the thread owners or other viewers.
Sometimes accounts crop up and stand out from the beginning, which leads to them being merited easily without a wink. I’ve seen a few of those recently.
Ux is one of such accounts: The account was created three days ago, and already received 10 merits for 9 posts (he’s done something weird though, and seems to have deleted the content of some of his merited OPs for some reason).