What things make appendicitis more likely?

Nothing except. Having an appendix is what makes it more likely altho many people live to ripe old age without ever having to have it removed. There is no prevention that one can do to avoid appendicitis.
Age. Appendicitis is a common disease. It is most common in teenagers and young adults - very unusual in babies and very small children and very old people. Interestingly, it is more common in young men than in menstruating women. Pain in the mid or lower abdomen that moves to the right lower abdomen is classic, but not all appendicitis has the classic symptoms.