stab me behind my back because I don’t fucking know.

Because that's their nature. Because they want to. Because they can. And because they are excellent at telling various reasons for themselves and others why it was necessary to do. It's a good thing if you spot a backstabber during something that eventually wasn't that big of a deal. Early. It's the worst thing if you find that out when you are in the worst situation ever. Or even "better", the backstabber has created that bad place for you.

But hey, if some people need lots of friends, and have a craving to be superficial, that's their choice. It would just be awesome if those people would carry a visible sign to warn people who are real and sincere. Like a tattoo on their forehead or a huge sign hanging on their neck. :D