Delusion does come into it. As had been stated before, suicide is not typically the result of a rational thought or decision process, particularly in the case of depression. Bear in mind here that clinical depression is a disease, a mental health illness, so while your comment regarding insanity is innacurate, it is at least within the right ball park.
By defintion, a suicide *is* usually a slefish act, but the motiviation is not selfish. Again, as has been stated, the suicide is only considering their own pain, and the personal consequences of their decision, because that is all they are capable of. They lack the perspective to even consider how their actions will impact others, or have a completely skewed perception of that impact - 'they'll be better off without me", "they'll suffer less with me gone", "they don't love me, so they won't miss me", etc, etc, etc.
Completely self-absorbed is perhaps a better description than selfish really.
So far as non-depression related motivations for suicide? Off the top of my head I can think of two, acute shame and desperation, when combined with a lack or failure of coping mechanisms.
The (rare in this country) criminal who kills himself to avoid capture by police for example, or the young girl from a conservative culture who finds herself pregnant.
Also, evidently in Zimbabwe there was a phenomenal increase in geriatric suicides due to the economic collapse - suddenly their pensions couldn't even buy them food, let alone cover any other expenses, with the result that rather than starve, a large number of them commited suicide.