what makes (one) tick
The things that motivate one or make one behave in a certain way. I'd love to sit you down for an interview so I can find out what makes you tick. Janet is a very private person, so it's hard to know what makes her tick.
what makes one tick
What makes one function characteristically, what motivates one, as in We've never figured out what makes these chess players tick. This expression alludes to tick in the sense of "function as an operating mechanism, such as a clock." [Colloquial; first half of 1900s]