What affects someone's mental health?

Everything: You are the total of all of your biochemistry and experiences, both good and bad. Therefore, everything affects your emotional or mental health.

Everything, Mostly: You name it, mental health is affected by just about everything, including your genes, experiences, gender, age, how much money you have and much more. Rest assured, no one thing is going to doom you to a life of mental health struggle. And there are many things you can do on your own or with a professional that will help improve your mental health throughout life.

Perception: Genetics has the biggest influence and it is the dye that colors the totality of ones life. Perception (which is impacted by ones sense of resiliency, temperament and ability to have healthy attachments) also has heavily impact ones mental health.