what makes someone tick

what makes (one) tick

The things that motivate someone; the reasons why a person behaves in a certain way. I'd love to sit you down for an interview and find out what makes you tick. Growing up hearing about her grandfather's heroic deeds in the war are a major part of what makes Janet tick.
See also: make, tick, what

what makes someone tick

what motivates someone. informal
See also: make, tick, what