motivation
noun
uk/ˌməʊtɪˈveɪʃən/ us [ U ]
enthusiasm for doing something:
Many staff lack motivation and are resistant to change.
[ C ]
the need or reason for doing something:
a motivation for sth A major motivation for his later career was to secure a regular financial income.
a motivation behind sth The motivation behind the decision was the desire to cut costs.
the chief/main/primary motivation