Does that sound familiar?
:) I don't know what 'being friends' really entails.
Being able to tell you on your face what you should or shouldn't do.
Remembering birthdays. :)
Going out together.
Does it take all of this? Or is it an implicit understanding? I'm around, and I'll be there if you need me. Is that enough? Can you hurt someone when you're angry/upset and not apologise later because they'd understand anyway? Can you not be in touch with one for months/years and meet as though no time had passed? Can you be annoyed with some habits and mannerisms of your friend?
Phew. Apologies again for the vagueness. A farrago of thoughts, and an attempt to bring into them some coherence.