Argument from Community
We are connected to each other, and it causes suffering of others when we choose to kill ourselves. We leave behind others who have to try and process the fact that someone they cared about has killed themselves, that the suicide was suffering to such a degree that they considered death to be a viable choice. This causes suffering that no one should have to go through. If it's unethical to cause unnecessary suffering in life, it is also unethical to cause unnecessary suffering by choosing death. I do find this compelling. One problem I see, however, at least from a psychological perspective, is that when someone is in that state, it can frequently be difficult for them to truly see that people care about them. How do you persuade someone to hold on, to keep living, if they believe that they are socially alone, that they have no community to speak of?
Argument from Influencing Others to Commit Suicide
In part because of our connection to each other, when we commit suicide it has a documented effect such that it can influence others to commit suicide as well (which Hecht goes into detail on). In Hecht's view, this makes it a form of delayed murder. I'm not sure that I'd go so far as to call it "murder," but I think I can still see an argument for a degree of responsibility. Although, that brings up an interesting question: to what extent can we be blamed, or praised, for the actions that others undertake through our influence? When I was suicidal in the past, I tried to keep it firmly planted in my mind that regardless of what forces I thought were pushing me, the ultimate decision would always be mine, and mine alone. I couldn't blame anyone else for it. I hesitate to make this comparison, but to some extent, this seems like blaming the victim of rape for the influence their actions had on their rapist. Yet, it seems to me that a rapist is completely responsible for his or her own actions, and there is no blame that can be assigned to the victim. So can you truly be blamed for your suicide influencing others to commit suicide? I find myself uncertain.
Argument from Future Self
People change. Depression waxes and wanes, as do suicidal thoughts. Our future self may not be the suffering individual that is currently considering suicide, so long as that future self is given a chance to live. By committing suicide, we deprive that future self of any future possibility of happiness, satisfaction, or any other good thing that may come. Me being me at this moment, I have to say that I'm very glad none of my "past selves" have succeeded in committing suicide, so this is an attractive bit of reasoning.
Hope. It's an extremely difficult thing to cultivate at times, and especially difficult to cultivate when in the midst of depression. The worse the depression, the harder it is to find any sense of hope. But we know that even some of the worst depression has better days, days that aren't quite as bad as the others. This fact alone can serve to give hope that there will be better days down the road. It may take time, and we may need help (in the form of therapy, and/or meds), but we can get there. If we kill ourselves on a bad day, then we deny all the good, or at least better, days to come.
Suicide used to be called "self-murder," before the days when it gained it's own word. I think that's an apt way of looking at it. It's murder of oneself (even if it isn't delayed murder of others), and murder of the self that could have been. For whatever reason it is wrong to murder another, it is then wrong to murder oneself. Even if we say that one of the reasons it's wrong to murder is that it denies the right of the other to make a choice, or actively goes against their choice, suicide is still wrong. Do we actually have the right to deny our future self the right to make a choice, any choice? The right to try and flourish?
In "Stay," Hecht quotes John Stuart Mill as discussing the possibility of someone selling themselves into slavery, which he considers as a null and void contract from the beginning (p177-178):
The ground for thus limiting his power of voluntarily disposing of his own lot in life, is apparent, and is very clearly seen in this extreme case. The reason for not interfering, unless for the sake of others, with a person's voluntary acts, is consideration for liberty. His voluntary choice is evidence that what he so chooses is desirable, or at the least endurable, to him, and his good is on the whole best provided for by allowing him to take his own means of pursuing it. But by selling himself for a slave, he abdicates his liberty; he forgoes any future use of it, beyond that single act. He therefore defeats, in his own case, the very purpose which is the justification of allowing him to dispose of himself. He is no longer free; but is thenceforth in a position which has no longer the presumption in its favor, that would be afforded by his voluntarily remaining in it. The principle of freedom cannot require that he should be free not to be free. It is not freedom, to be allowed to alienate his freedom.
If we cannot sell ourselves into slavery because it denies us our very liberty to choose, then we surely cannot be justified in suicide, which not only denies the ability to choose, but the ability to change our mind.
One more point. Though I would never wish on anyone the suffering that can lead one to contemplate suicide, and indeed would wish for the very opposite, given that people do experience such suffering, I would like to point out the silver lining in the rather dark cloud. If we can learn to get out of depression and past suffering, or even to just find better days within the depression and suffering, then we can become stronger. We can take the lessons we learn, and use them to be stronger in the face of our own suffering. And as we become stronger, we will find that the better days are even better, and, going back to the theme of hope, we just may come out the other side a stronger person who suffers less.
At least, that's how it's been for me.