Well, they were probably genetically predisposed to dopamine sensitivity. I don't know enough about it to know how it comes to be, but it basically involves the expectation of rewards and sensitivity towards it.
Let's say you wake up today expecting a 5 from the world, and you suddenly won the lottery, an obvious 10. If you wake up tomorrow expecting a repeat performance, a 10, it obvious isn't likely to happen. Some people have better hormonal regulatory system than others, thus they can reset back to 5 the next day. Some people are more sensitive to outside influence in this regard, so they will be expecting a 10 for quite a while before they can reset themselves.
This type of sensitivity can really wreck havoc on someone's life if one combines it with certain home environments.
Due to popularity of pseudo psychology, a lot of patients are quickly labeled, judged, and promptly misdiagnosed. Words like depression, Prozac, ADHD are as plenty as tap water. Is that really helpful or useful? Misinterpretation is worse than not been able to understand it at all.
Psychology/Neuro-psychology/or whatever names you call it, is a difficult subject. After the genome, proteome, and physiome databases are established, maybe then we will have enough resources to create a psychome database. An actual empirical study of the cause and effect of the human psyche. Of course, that isn't going to happen any time soon.
In terms of suicide, it isn't as easy or simple as it seems. As a biological organism, our instinct/or biological determination to live is very strong. Of course, emotional flooding can cause the whole ecosystem of the brain to go haywire, but that usually isn't enough rationalization for someone to "off" him/herself. Actually, if someone digs deep enough, the underlying desire isn't so much escapism, it is more likely to be an desire for freedom. Only when someone "feels" he/she has nowhere to go, no-one to turn to, nothing else left, this will be an option for him/her. That is why near death experience is so powerful. The close and intimate struggle with death, and the subsequent revival of the will to live helps someone get past all the rationalizations he/she has given his/herself in order to live with less pain, then break through from it. A lot of older martial arts training methods involves something similar to this. (i.e. standing or sitting under the waterfall) Frankly, it isn't always easy to be honest with one's own self. We make white lies to live easier lives. But when it piles up, it will conflict with one's own sense of reality, and the end result can be disastrous.
In my own pov, no matter what from it takes, death, in the end, is just lonely. The desire to have others to acknowledge your existence, your worth is eternal. If you use the termination of your own existence as the last testament of yourself, in order for others to lament what could have been, is in the end, self defeating. And if there is a after life, you are still lonely.