Man is a social animal. Early man realized this that he cannot live without a companion or a friend. Besides they were more productive hunting in packs. In the same manner, we can be more productive and more informed through the experiences and ideas of others.
I agree with Gypsy that there are times we may need to be alone and do our own thing.
And as JoeKanuck says friends are there when we want an interest-free loan. :)