Why do we need friends?

Why do people need friends? Friends will always be nice in simple problems and allow you to have some of their cookies or give you advice on how to do a manicure but when a big problem arises friends will just do what benefits them - that is just human nature to think of yourself before others! If both of you were stuck in a faraway country and both urgently needed to get home but only had 1 plane ticket between you the friend would try to take that ticket herself and wouldn't offer it to you. Or if you both had an important exam coming up and you needed her help she would revise to get an A+ rather than an A (without revision she would get A, with revision she would get A+) rather than helping you even though you missed half of the term because you were ill and without her help you would not ge higher than a C. Whereas family members like cousins or sisters are always there to help you. They won't put their own needs bscore your own, they will consider both equally. Basicaly, what I am saying is, why do we need any other friends other than family members? Non family will be there to share happy times with you but when there is a big problem they will only think about themselves. Just from a psychological point of view - it is human nature to put yourself before others but not family.