According to a Gallup poll done about a year ago on the moral acceptability of certain practices, 82% of Americans find suicide to be morally unacceptable and 18% find it acceptable.
This is a huge difference, and one I would attribute to the influence of Christian thinking if I could find a Biblical passage that condemns it, but I can't. So, is this more of a secular position? If it is, what are the secular reasons for scorning suicide?
Most humans love life and hate death. Some humans hate life and love death, these commit suicide, and they are in the vast minority.
So it is fairly natural for the members of an opposing ideology to dislike those who reject their ideal (life is good, death is bad) in favor of the inverse (death is good, life is bad).
If you didn't think death preferable to life, you wouldn't commit suicide. And so the act of suicide is an act of rejection to a deeply held belief of most peoples, even most atheists, that being the belief that life is good or desirable, and death is bad or undesirable.
So overall the rejection is based on a fundamental moral axiom that almost all humans, religious or secular, hold, consciously or unconsciously. As suicide is by definition a rejection of that moral axiom, most view it as a bad.
And there is a third group, those who have been "freed from the bond" in the Taoist sense, who love neither life nor death and hate neither life nor death, but accept both as they come and go, and as the cycle of change goes onward. For the third group, the rejection of life that the suicidal person exhibits is folly, but then again so is the clinging to life and fear of death that the majority of humans exhibit.