well... all behaviour has a biochemical cause
(animal spirits and soul stuff went out last century)
since the brain causes behaviour
and the brain processes are biochemical in nature
your behaviour is caused by biomedical processes
just like everybody else...

if you want to teach someone how to cook a curry you are going to have better luck altering their biochemical processes by showing them how than you are going to have luck altering their biochemical processes by medication or psychotherapy.
just because your behaviour is caused by biomedical processes doesn't mean medication is the best way to alter your biomedical processes

as for whether mental disorders are caused by MALFUNCTIONING biomedical processes or not is an issue that is currently the subject of much controversy.

people do say the darnedest things...