As a human being, one can hardly do without a friend. In the US state of Pennsylvania, there used to be an auto license slogan, "You've got a friend in Pennsylvania." This slogan became a wide spread joke about people who lived in Pennsylvania, and people from other states would roll down their car windows and ask the driver of the Pennsylvania auto, "Are you my friend?" Eventually, Pennsylvanians got tired of this joke, and now the newer auto license plates for the state have eliminated this slogan.
Do we need friends? Are you my friend? What does it mean to be a friend? How many close friends do you have? How many do you need or want? Do friends change over time? Are there different classes of friends? 
No doubt, our best friends are those who are family members with whom we have a life long experience or others with whom we have had very good relationships over a long time. These are persons from whom we can almost always seek assistance, advice, or help. We all need such deep friendships as they develop the main fabric of our lives -- a sense of belonging to a special community. I have heard people refer to someone as "a friend for life." Life long friends have certain important responsibilities to each other, which surpass more casual friendships. However, the number of our very best friends in our lives remains reasonably stable and small. Many Americans have friendships which are situational or useful. Because of great mobility in American society, when students complete a class with a special friend and change classes where they are no longer together, or people change jobs, or when people move to new locations, they often lose complete touch with the former friends. Then also, Americans have many acquaintances or colleagues who would not precisely be called friends, but with whom we are friendly. Even in terms of romance, many young Americans often develop a series of short-term rather deep relationships which may dissolve without much future contact in the future.
In many societies, including China, real friendships are usually much deeper and are built up over a longer period of time than for Americans. International students or visitors to the US or who meet and become friendly with Americans in China often find themselves quite surprised and disappointed when the friendship doesn't develop to a deeper level, or ends rather abruptly. The Americans might see such friendships as situational or the friends as casual acquaintances, while those from other societies may mistakenly see themselves in a deep and lifelong friendship relationship.
It is important to know what friendship means in different cultures and societies, and to adjust to their explicit and implicit aspects of friendship. But, are you my friend? Well, perhaps! If we can depend on each other "in thick and thin, n then we surely are real friends, but if it is a situational or simply useful friendship, then you might want to assume that over time things will change and that old friendships may dissolve or new ones might develop.