I hear ya, but like, the only way you know if someone is gonna be a good friend is to let them in, at least a little.
If it doesn't work, you can part ways; if they betray you, you can part ways faster.
But if not, maybe there's a good person in there that would make a great friend.
Also, I've always heard that "The best way to make a friend is to be a friend," but that phrase has been mostly useless to me. Maybe someone else can find some purpose in it.