Why do we say we will be friends with someone? Where does that “s” come from? It’s not plural or possesive, no?
Actually, it is a plural. There are two people involved, you and someone else, because you are with someone other than yourself. Therefore, you are both friends. The singular would be “I am a friend to (or of) so-and-so.”
It’s like saying “He and I are partners.” A partner is only one half of the equation; taken as a pair, you are partners. Hope this makes sense.