It's not logical and it's not something you can really "understand" because it doesn't usually make any sense.
I think anyone can get to that place if you put the through the "right" life circumstances and issues, but some people just seem to be born with a propensity for it. Also depression taken to extreme is not just feeling down and depressed, it can become a psychotic state and then all bets are off.
While some people might "want" to commit suicide, I don't think it's usually about wanting it, quite the opposite.
Also it's not always strictly mental and the line can be blurry, like if you're experiencing depression as an aftermath to concussion (ie. brain damage), is that mental or physical or both? Lot of people suffering from CTE end up killing themselves, like the hockey player mentioned in that article and often it's only discovered while performing autopsy that they had CTE.
Depression doesn't necessarily have anything to do with the severity of life shit you have gone through, it can be an insidious thing that slowly creeps on you, extending it's tendrils until one day they have you in their grip and you find yourself in the depths of depression, with no apparent reason. Judging yourself for being depressed for "no reason" is about the worst thing you can do in that situation and why it's important to get rid of the stigma around mental illnesses because the longer you try to just supress it, the worse it tends to get because the wall between your inner experience of life and what you show to other people gets thicker and thicker, until you are trapped in an isolated hell of your own.
Also we are emotional and social animals, we are connected with each other so one way to look at it, which I think might be sometimes helpful, is that the individual suffering from depression is just the symptom of a problem that doesn't necessarily begin and end with that individual alone. I believe that if everyone was perfectly in touch with themselves and their emotions, able to accept themselves and live their life true to their heart, no one would get depressed unless they had brain damage, some serious hormonal imbalance or other physical reason that can be tracked down to a malfunctioning organ.