To those of you who argue about the rationality of such a decision: I do personally agree that it's irrational. It's usually emotion-driven, sometimes fueled by instability, and rarely by a careful analysis of pros and cons. Of course, exceptions to that exist (and are probably more numerous than they get credit for), but it's a fair enough generalisation to make.
That said, western society currently claims to believe in a person's freedom to make their own decisions, rational or not. Because of that, I don't think we should focus on preventing suicide, but merely on ensuring that it doesn't screw with other people. Now, I'm fully aware that this approach has many fundamental flaws and couldn't be implemented as-is. If it were, we'd probably see chains of suicide, with one person discovering the other and, in the heat of the moment, also committing suicide. I certainly don't have all the answers here, but I do firmly believe that "it's irrational" is not a showstopper.
Many thanks to those who weighed in on the subject so far. While reading more (which, admittedly, is mostly restricted to shitty niche blogs, since the subject turns out to be somewhat taboo), I discovered a point of contention: suicide notes. Some argued that they should be thorough in explaining one's decision and trying to make sure that nobody's left with haunting questions and a feeling of self-blame; others suggested that such a note may be read by people long after one's death and bring back some haunting or traumatising memories. I'm not sure where I stand on that, but I think I'd be leaning towards the long and thorough note.