Apoptosis and Suicide
'Suicide is contrary to biology' is an oft heard statement, expressed in different ways. This statement, unqualified as such, is somewhat erroneous, because suicide is, in fact, a very common phenomenon in biology at the cellular level. It is known as 'apoptosis', programmed cell-death. In the natural state it occurs only for the development and preservation of the organism. And if we are to believe the positivists of the 19th century, society is the next step in the integration of life, and society assumes a role akin to that of an organism, and individuals that of a cell. Hence, we also find programmed individual-deaths for the preservation of society (kami-kaze, for instance). Apoptosis which is not for the purpose of preservation of organism is triggered as a result of some pathology, due to some injury to the cell. To anthropomorphosize, the cell shouts "I can't take it anymore!!" and pop goes the apoptosis. Similarly suicide which is not for any higher purpose, is a result of some psychological injury to the individual. The human also yells 'I can't take it anymore!!' and does something self-destructive.
Another parallel could be drawn. Some cells apoptose because they were inherently abnormal (had some DNA defect, for example) and it is nature's way of weeding them out. Now, what i am going to suggest is purely theoretical, and is not an established scientific fact. Some individuals commit suicide because there is something inherently defective about them. Some genetic basis for depression or bipolar disorder, maybe. And they are drawn to suicide just like an abnormal cell is drawn to apoptosis; nature's way of weeding them out.