It's experience,
friends are by choice.
acquaintances have association.
To me it seems to be a judgment call.
A stranger judges you and you have no idea.
An acquaintance judges you but you don't know why.
A friend judges you and let's you know it with an explanation.
Of course this is all on theory, not that I judge people,
discerning differences have a different motive,
acquaintances have something in common,
friends have more in common,
Care, and time.
To me, friends are people who have agreed to the permission of judgment, and regardless of the verdict, it's not a one-way motive. Acquaintances share thoughts, while friends share feelings. Acquaintances can only assume what friends already know by experience and common beliefs. Truth is, camaraderie is found in both, acquaintances will work by you, whereas a friend will work with you.