Hi Sterlin,
Thank you for taking the time to address this very important issue. As someone who has been "diagnosed" or "labeled" with mental illness, I think that I am well qualified to speak to this subject.
First, let me preface this by saying that I am in agreement with you that there are far too many prescriptions being handed out today for things we label as "diseases" or "disorders." This is a direct result of Big Pharma marketing.

As to your statement that, "nothing is wrong with them, aside from feeling down or unloved, " I'm not sure how to respond to this statement except to say that it lacks the same scientific merit you are attempting to prove doesn't exist within the mental health field. I can assure you that "feeling down or unloved" isn't the cause of depression for me. I am very loved and I feel appreciated by my friends and family.

As far as the brain goes, I will give you an example.

Say for example you take some benadryl. This causes your brain to block the action of histamine. Now, the same thing can happen when a person eats toxic food such as wheat because it is sprayed with Glyphosphate. Glyphosphate can interfere with a person's gut bacteria, which also contains neurotransmitters, so it isn't just the brain that produces neurotransmitters such as dopamine and serotonin. Once the gut bacteria is altered, so is the brain. Depression can occur as a result of eating too much junk food, particularly sugar. To say that depression is a result of thinking "nobody loves me" completely misunderstands the powerful impact the GMO food has on the brain.
To that end, I must now comment as someone who has lived with the label of Borderline Personality Disorder. I can assure you that those of us who suffer with Mental Illness certainly don't perceive it to be something that doesn't exist. Mood swings and intense anger are just some of the things experienced by a Borderline. People diagnosed with BPD have difficulty regulating their emotions and the root cause of this can be found in Childhood Trauma and Stress. Dr. Gabor Mate explores this issue in those who have Substance Use Disorders. HIs book, In the Realm of Hungry Ghosts addresses people who use drugs and alcohol to self-medicate. It has been found that the brains of people with early childhood trauma/stress have structural differences that make it difficult for them to process emotions in an effective way. In other words, they haven't learned how to self-soothe. That being said, there is a paper I would like to refer you to titled: Structural and Functional Neuroplasticity in Relation to Traumatic Stress.

The following quotes are from said paper:

"The body’s stress response is an essential
adaptive and protective mechanism to cope with threatening
situations. However, chronic or traumatic stress
leads to structural and functional alterations in the
traumatized brain. We argue for a building-block effect:
Exposure to different types of traumatic events increases
the probability of developing posttraumatic stress disorder
(PTSD), via incremental enlargement of a fear network.
We summarize evidence of brain changes in PTSD,
including recent results from research on animal models of
stress-related neuroplastic remodeling, with an emphasis
on structural and functional changes in the hippocampus,
the amygdala, and the medial prefrontal cortex."
The paper goes on to say:
"Traumatic stress refers to potentially harmful experiences
eliciting feelings of helplessness, intense fear, or horror, with
an associated alarm response (cf. Elbert, Rockstroh, Kolassa,
Schauer, & Neuner, 2006)—that is, the acute release of stress
hormones. Each such experience is appropriately referred to as
a trauma (Greek for ‘‘wound’’); traumas render a person more
vulnerable to develop PTSD in a cumulative manner."

I would also like to add that many NFL players end up with Post Concussion Syndrome from a Traumatic Brain Injury and end up committing suicide because of it. According to headinjury dot com: "PCS is a specific set of neuropsychological (thinking, behavioral, and emotional) disorders caused by traumatic brain injury, aka concussion. PCS results from actual, physical, damage, or injury to the brain caused by an external force. A brain subjected to such violent forces can be torn or sheared, crushed, or displaced, or simply destroyed. It can bleed, swell, and occasionally, it might even shut down. The resultant condition is known as traumatic brain injury, TBI..
Leading causes of TBI are motor vehicle accidents, work place injuries, acts of violence, falls, sports and recreational injuries. TBI can occur without any outward physical evidence of injury or trauma. Examples of such invisible injury include, whiplash and shaken babies."

As you can see from this description, there are people who suffer from depression as a result of a head injury and often it is misdiagnosed.

SOURCES:
http://kops.uni-konstanz.de/bitstream/handle/123456789/11234/1/Kolassa_Elbert_2007.pdf
http://www.headinjury.com/faqpcs.htm