I have always known since I was five that death is a basic thought that comes in everyone's mind. People hate their lives sometimes, they just want to die. It's simple, isn't it? No, because there is something more to our tendency of hating our lives not everyone sees...
Strong Exterior
Now, our strong exterior is a dead layer. I hate to admit it as a person with a hard shell myself, but it could really be pretty lifeless in a certain level. Some people forget about balance that they never allow themselves to strip free from that tough cover at least once in a while. Also be warned that this tough exterior does not only come as a literally strong character, for it also comes with an extremely soft facade that is actually utilized at its maximum as a form of silent weapon. Now, people with this delicate nature are usually the ones who find it difficult to just lay bare that they miss out on the pleasures of life that keep a person alive.
Basically, being tough and tender should always come together. When they don't, those who are too tough feel like death is the only solution to their lack of tenderness while for those who are too soft, they wish death upon themselves living the life where they choose not to brave the wilderness out there. Therefore, our perceived emptiness in life is not emptiness at all but rather an imbalance in decisions we ourselves make.
Death's Role
Life is all about that imbalance, and death has always given us the idea that all ends lead to new beginnings. Take the Christian view of death for example--you die and you either go to heaven and live happily or go to hell and still live but with endless suffering. However, death still remains a mysterious concept despite the ideas engraved in this society by the widely known Christianity. That space for skepticism is what makes each of us suicidal.
Meanwhile, death could be just anything a person perceives it to be, thus it is easier to invest on it. To hell with the complicated pursuit of balance when you're already dead on the inside anyway, right? Still, what-could've-been's are a strong influence as well, so a lot of us still choose to live.