Why do we trust our friends?

Bismillahirrahmanirrahim.
Assalamualaikum warahmatullahi wabarakatuh.

Just wanna show you this.

Notice Sasuke's (the one below) reaction? He looked like he's in disbelief.

He actually couldn't believe that Naruto (the upper one) would say that, although Sasuke was going to kill him and the whole village soon.

You see, Sasuke is the bad guy and Naruto is the good guy. But Naruto and Sasuke are good friends. In fact, Sasuke's the only person that Naruto has is his world. So thus Sasuke. But Sasuke's hatred has swolen up as time passed. With his whole family killed, based on a cruel reason that is to protect the village, he himself turned to be the greatest enemy of his own village. And Naruto, whose ambition is to be the greatest head of the village, or the so-called Hokage, the greatest shinobi/ninja in his village, would have to make a heartbreaking decision, that is to kill Sasuke.

But as the love towards Sasuke is too strong, as a friend, Naruto had finally decided. In the battle between him and Sasuke, he would make sure that both of them would be killed.

But here, actually I would be very lucky to be Sasuke. I mean, isn't he lucky enough to have such a loyal friend? And I know the fact that men's friendship would amaze a woman. I wouldn't say that women make less strong friendship, but just admit it that guys do it better. And I also admit that I am very amazed with this, as I never, in my life, experience such a priceless friendship, the one that has no blood ties, but even stronger. With no other bond between the two people except 'friendship'.

This really amazed me.

What is the feeling that allow humans to go to the extend that trusting people who is not your family members?
Well I agree that not all family members are good and can be trusted, but well, they are our family though.

Why would humans do this to a person, before, a stranger to us? Why do we trust our friends?

*Btw, to all my friends out there, please know and be sure that you would always be my friends. And you are all my Narutos, because I am Uchiha Sasuke ;) {Sasuke is so cool~ =P}