Friends are actually a detriment in a world where everything is trying to kill you. Ever notice how the lone survivor always gets closer to dying whenever he gains a companion or two?
With friends you tend to let down your guard more and trust in them to get things done, which they always screw up of course. Then you're always worrying about if they can handle themselves in a fight, trying to save them if they get captured or hurt, allowing them to eat your food and drink your water, and make friends with your dog.