Good question....
I agree - genes, body chemistry and environment. All of these play a role.
Genes play a role as research has proven that if one or more parents have a disorder, then our chances increase for us to have the same or a similar disorder.
Body chemistry plays a role as for example - serotonin levels are not where they should be. Low Serotonin levels are believed to be the reason for many cases of mild to moderate depression which can lead to symptoms like anxiety, apathy, fear, feelings of worthlessness, insomnia and fatigue.
Environment plays a role as for example- coping skills for learning how to deal with a particular situation are not shown to us when we are young.
I do think that all of these factors contribute to our mental well being.

Heather
"In three words I can sum up everything I've learned about life - it goes on."
~~Robert Frost