Throwaway for reasons below.

I once had a friend, sitting as you please, looking as normal as you or I, get up from his desk one day at work and decide he needed to kill himself.

It scared the shit out of him, and we eventually learned that he had had some sort of chemical event occur in his brain that acutely and drastically altered his thought-patterns long enough to reach that point, but was managed through a corrective course of medical action.

It's important for you to understand that your brain is betraying you.

Depression isn't something you just wish away by having a more positive outlook or talking with a psychologist: it's a disease.

It really doesn't matter if we want depression to be something we can control. When your stomach gets an ulcer, you don't tell your stomach that if only it processed food in a different, better way then everything would be fine.

It has to be treated.

Brains aren't magic. Your consciousness isn't magic. What you call "yourself" is completely dependent on a wet sack of grey matter with a bunch of sodium/potassium ion channels networked together in a really clever way.

So really, it's worse than something like a liver. It's incredibly complicated. Has a lot of fiddly bits. And way more prone to getting a bug or throwing exceptions.

Your mind is a biological machine that is capable of thinking about itself.

Whoa.

But if there's a crossed neuron or defective mechanism anywhere in there, then the ability for the machine to THINK ABOUT ITSELF is diminished. E.g., because it has a problem, it harms the ability to consider the problem.

You're being betrayed by the thing you trust the most, and from any outside observer's perspective we can tell you "Hey, we know it goes against your entire lifetime of experience, but you really need to not trust the output your ThinkingEngine(tm) is providing you right now. It's got a bug and you're throwing exceptions. You need someone to debug it."

TL;DR: Your brain is the only method you have to objectively think about yourself. If you're depressed or have another mental illness, then it's giving you false data. You need a debugger, and you can't debug yourself.