What caused Mikey Way's depression?

There is almost never a single cause of depression. It can start due to anything from a hormonal imbalances, drug use, medication use, stress, genetic predisposition triggered দ্বারা any number of factors etc etc. There is no universal cause.

In any case, Mikey ব্যক্ত he was diagnosed with Bipolar Disorder, which is not the same as clinical depression...