Do atheists believe in the existence of friendship?

[KerimF]

First, thank you all for taking the trouble to answer my first question by a big YES, though each of you did it by using his artistic talent

Now, it is time for the second question

If two persons stand before you, please let me know how you could be sure that friendship exists in them or not. Thank you.

Naturally, the third question will be based on how each of you will answer this second one

Friendship is the name we put on a series of behaviors and feelings demonstrated by humans toward each. And lack of other behaviors that one associates with not being friends.

Of course friendship exists, because we can see people demonstrating those behaviors toward each other.

If two people are standing before me, and they are demonstrating most or all of the behaviors we associate with friendship, are avoiding the behaviors we associate with not being friends, that is pretty good evidence that they are friends.

For example, if I see them help each other out, go to movies or concerts together, wish each other the best in life, become sad for each other if they are going through tough times, compliment each other, if one chooses the other to be her bridesmaid or best man at their wedding, etc, etc, etc, since that all is part of some of the collected behaviors we associate with friendship, as a third person observing them, I can be pretty sure they are friends.

Could I be wrong? Sure.

But the evidence is pretty compelling they are friends.

And yes, love, friendship, empathy, sympathy, dislike, etc, are all a result of brain function, and brain chemicals.

All you said about friendship is indeed true (and interesting). But I am afraid you missed the most important thing which, if accepted by both sides, their friendship becomes perfect. This thing has nothing to do with chemicals or feelings though almost all people look for it.
In fact, as we will see, the answer is known already by almost everyone here, for being obvious. So, I am surprised how no one mentioned it yet!