The main neurochemical associated with Depression is serotonin , however , dopamine is associated with negative symptoms of depression.

Serotonin plays a predominant role in depression. Sir. Dopamine is responsible for the negative symptoms.

Yes could be many reasons

Low levels of dopamine can cause depression. However depression is a multifaceted disorder with many neurotransmitters(NT) implicated in it. Serotonin is one of the most commonly depleted NT along with dopamine and norepinephrine. This is just one factor for endogenous depression, external causes also play important role like interpersonal stressors, job satisfaction, financial condition, friends etc.