Why the gene for suicide?

It is a puzzle why evolution doesn't eliminate the gene that gives people propensity to commit suicide, because if someone has this "suicide" gene and does commit suicide, this gene would not pass on. Natural selection should have selected against that gene, but as we can see, natural selection is not that successful in doing this. We can see some suicidal or nearly suicidal behaviors among animals, which can be explained by kin selection: when your suicidal behavior benefit your close relatives, then it may be helpful to pass on the copy of genes on your relatives, which are the same as yours. The suicidal behavior of human, however, seldom benefit their kin, and hence cannot be explained by kin selection.

To solve this puzzle, it is helpful to realize that not every characteristic of human being is selected directly by natural selection. Sometimes they are just by-products of other characteristics. I would argue that suicidal behavior and its major cause depression are by-products of the human desire to be significant and to have a purpose. This desire to be significant is a major driving force for human, which can bring them more resources and more mating opportunities. When we fail to achieve what we planned, we feel sad. This bad feeling motivates us to try harder. This close to perfect design in the hunter-gatherer society, however, is flawed in modern society. We now live in a time where there are tons of situations that can put us into stress, tons of reasons for us to get disappointed and the fast paced living keeps us from recovering from the stress and disappointment, hence depression develops. As suicides are almost unheard of in primitive societies, we can say that human suicidal behavior is a product of the mal-adaptation of our genes to modern environment.