One of the first theories relevant to understand why people buy has been the “Theory of Human Motivations“ presented by the psychologist Abrahm Maslow.
It dates back to the 1943 but is still the basis of present-day Marketing.

The basis of Maslow’s theory of motivation is that human beings are motivated by unsatisfied needs, which exist in an hierarchical way so that, normally, human beings tend not to be concerned about higher levels of needs unless they fullilll the lower ones (you don’t really think about your haircut when you are starving...)
There are general needs which have to be fulfilled before a person is able to act unselfishly. The satisfaction of these needs is quite healthy, while preventing their gratification makes us ill (like anxiety, anorexia, complex of every kind) or act evilly (selfishness, envy...).

Identity Manual
Survival needs are obvious.They are the literal requirements for human survival.
If these requirements are not met, the human body simply cannot continue to function or preserve the human species.
These Needs are dealing with achieving of stability and protection in a chaotic world.
Things like having a roof on your head, the safeness of your family and your person are primary goals at this stage of needs.

Human beings need to feel a sense of belonging, acceptance and identity, whether it comes from religious groups, professional organisations, sports teams, social connections or, most important, the family.
All human beings need to be respected, to have self-esteem and self-respect.
Esteem presents the normal human desire to be accepted by others, gain recognition and to feel self-valued, be it in a profession or life.
“What a man can be, he must be”. 
This forms the basis of the perceived need for self-actualization. This level of need pertains to what a person's full potential is and realizing that potential.
The need to connect to something beyond the ego or to help others find self-fulfillment and realize their potential.
In other words, you can not reach the real happiness unless people you love are happy too.

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