And why does depression cause physical symptoms aswell as mental symptoms?

Some physical symptoms of depression are…

Unexplained aches and pains
Sleeping too much or too little
Eating too much or too little
Headaches or stomach aches
Dizziness or light-headed-ness

These symptoms are caused simply because having a negative attitude and being sad all of the time can take a toll on the brain. It just kind of drags your physical health down into the same dark cyclone that your mental health is going into. People who are happier are less likely to get sick, and recover easier. Depression is not likely to make a person ill with anything other than depression, but these symptoms are still very unpleasant.

read this one
66.102.9.132/search?q=cache:Ym4WH…

here are some e;g:

Symptoms associated with depression include joint pain,
limb pain, back pain, gastrointestinal problems, fatigue,
psychomotor activity changes, and appetite changes. In
the primary care setting, a high percentage of patients with
depression present exclusively with physical symptoms.

this one from PDF file you can doenload it from psychiatrist.com/pcc/pccpdf/v…

all the best!

The only physical symptom was insomnia. I was getting 2 to 4 hours of sleep for about 6 months till I started an antidepressant and counseling.
It took me another year of insomnia before my sleep started to get to normal.