What makes people click on a button?
Curiosity, FOMO
In short, people click when what they SEE matches what they WANT. But what they want can depend on other factors. Maybe they want to be in the know or want to do what others are doing. Some people even want things without consciously knowing why.
Should primary CTA be on the left or right?
Full page designs should have the primary button on the left side of the page. If the browser window is large, or you have to scroll down to fill out a form, it's best to have the primary button right below the last bit of content on the page.