What makes somebody tick
Definition 1 of 1
Pro Tip 1/3Idiomatic Usage
It's an idiom used to describe intimate understanding of someone's deeper motivations or interests. 
As a novelist, she delves into what makes her characters tick.
Pro Tip 2/3Personal Insight
Often used when discussing personal relationships or connections, emphasizing a desire to understand someone at a profound level. 
He's been my friend for years, but I still can’t pin down what makes him tick.
Pro Tip 3/3Colloquial Expression
It’s informal and commonly used in casual conversation rather than formal or academic writing. 
In casual talks with my mentor, I try to learn what makes her tick.