What causes sadness and depression?

[anagogy]

But why do I feel sadness and depression?

Is there a metaphysical reason?
Or is it a chemical imbalance in the brain?

I think you simply long for connections that 3rd density isn't really set up to facilitate. A common malady of wanderers (the desire to 'go home'). Thus, while there is both a mental, and physical component to your sadness, it is predominately a spiritual cause in my opinion. The mind reflects the inpourings of the spirit, and the body reflects the inpourings of the mind.