Depression is most certainly a thing, and is definitely not just "intense sadness". Deficiencies of serotonin and other similar neurotransmitters in the brain not only obviously decrease feelings of happiness, but also well-being, motivation, and energy while increasing anxiety, cynicism, and fatigue.
This is pretty much proven given the fact that SSRIs work for thousands of people, and on top of that, observing the effects of anti-depressants.
Most people I know say I shouldnt be so negative/depressive but thats how I've been for the past 4-6 or so years so I its a normal thing for me. Ive been thinking of getting tested but I simply don't believe a psychiatrist can get me diagnosed with a few questions. I guess measuring dopamine is a great way since its actually scientific but then again I haven't felt excited for something in these same 4-6 years. Maybe because I actually am depressed 🤔
I actually got a dopamine tat because I believe I have a lack of it. Altho 25% of the reason I got it in the first place was because I think it looks cool.
Low dopamine definitely can cause low motivation, but depression deals more with serotonin. However, problems with dopamine are currently the most accepted explanation for people with ADHD.
Again, that's why stimulants are the absolute most effective treatment for ADHD/ADD. Giving you that boost of dopamine does wonders. I should know, I'm currently taking Ritalin LA for mine.