If the one wishing to commit suicide is a family member, and you seek to stop them to prevent the grief it will cause among your family. That is a selfish action. You are thinking of yourself and others and not of the person in front of you. Laying the guilt of derived grief on them as a reason to stay their hand is disgusting and cruel and will not benefit them at all.
If the one wishing to commit suicide is a complete stranger or a casual acquaintance, and you wish to stop them, consider your reasons. "Because it's wrong" is subjective and based on your own morals. That is a selfish reason. "But they're throwing so much away" is a desire to sway them to your way of thinking. That is selfish. "It'll play merry hell with traffic" is logical and humorous, but ultimately selfish.
If given the option of interacting with someone who is suicidal, you may want to make sure the person is genuinely resolved to the act. If this person is committed, then they would not mind waiting for a bit. They could take the time to put their affairs in order before leaving, i.e. settling debts, selling off property, leaving a will, saying goodbye. They would confront all those things they seek to flee, and with the imminence of their cessation consider their importance. You have helped them make the choice that is right for them. That is one way help them.
Suicide is the one consequence-free decision we can make for ourselves. To clarify: there will be no consequences for the one who dies (ignoring religion for the moment).
Is that not immensely tempting? Speaking as one who has contemplated this subject, and the act, on a few occasions, the thought of freeing one's self from the seemingly endless march of horror that this world can bring seems a rather easy choice.
Yes, there are many magnificent things in the world to behold and be part of. Though when one is contemplating death, those happy thoughts are stale and hollow. A colorful mask hiding the darkness of the world. In the long march of time, a single life seems to affect so little, and the idea of respite from the misery one may be experiencing is quite appealing.
Please allow me to help clear the air. For those of you commenting on this subject, please consider these thoughts.
If you are speaking from some perceived moral high-ground, you are a fool and ought to stifle your noise-making. You have no idea what you are actually talking about. You've never held the knife to your own throat and stood one twitch from release, despair urging you on, so your ignorance will help no one.
If you are speaking from a sentimental core that all life is special and should be preserved, you ought to hold your breath. The one you seek to help will not hear such words. They would be empty and even cruel.
If you are speaking from a stance that one less person around wouldn't be so bad, your nihilism is evident. Take solace in that and say no more.
If you have been there and come away. If you have found the will to live or the courage to strive on with a foot over the precipice of the ultimate temptation, then you have a better chance than any to help someone. Not to live and not to die, but to make the right choice.