Because life is actually good. That's not an opinion; unless it's a terminal disease, where the person will die no matter what, life is worth living once you find the right path for it, and people are killing themselves unnecessarily because they don't know this.

To make an analogy, it's like watching a baby crawl into a burning fireplace and doing nothing to stop it. If you take the baby away, you're not taking away their rights, you're teaching them (factual) right from wrong.

If you approve of suicide, then that's your opinion; (and judging from your comments, you do; if you would let suicide happen and believe things like "they have the right to kill themselves" then you approve of suicide) just don't expect to sway people to your side.