I think HappyCheeks puts it well. If I found out that an acquaintance had said something bad about me, I wouldn't be happy about it, but I could shrug it off after a day or so. If someone I thought of as a friend did, I'd be hurt.

There's never any reason for a friend to say nasty things. Even if you're disagreeing about something, or having misunderstandings, nastiness wouldn't even enter your minds. You'd both be upset by any discord, and want to make things right.