In his book Drive (and his immensely popular TED talk), Daniel Pink argues that motivation is largely intrinsic and that it’s roots can be found in the rich soils autonomy, mastery and purpose. We might summarize his point: “one choses to do what they do because they find meaning in it.”
While written with a focus on organizational leadership, this goes much deeper. Pink’s theory of motivation is like the “Third Viennese School of Psychotherapy”, Logotherapy.
In his moving work, Man’s Search for Meaning, Victor Frankl introduces us to Logotherapy. Through his experiences in Nazi concentration camps during the Holocaust, Frankl developed a belief that the fundamental motivator of individuals at any point of time is their search for meaning (as opposed, for example, to Freud’s focus on pleasure).
Neither Frankl nor Pink are able to predict what will motivate us or others, but if you’re trying to find meaning for either they’re a good place to begin.