Why is suicide seen so negatively?

For example, if a person was dying of cancer and wanted to be let go... people take it much better than a suicidal person from mental health issues.

It's been a huge wave of this "you are not alone" stuff, and it's more annoying than anything. It feels like it tries to cover up the reasons why people feel this way. Making life seem like some fairytale, and you do not want to be here as "bad."

I feel like honestly would be better in these situations, but I still wonder why suicide is treated so differently. Why is it wrong for someone to want to take their own life? It's not like they will regret it once they are gone. They are gone. It's a whole movement about "taking control of your own life" but not TOO far, I suppose.

Edit: Hey, I didn't expect so much discussion, I appreciate it. I want people to know that even though I'm saying this, I don't mean you SHOULD commit. I'm discussing the taboo around the subject of suicide, and how it's handled in (imo) a strange, backwards way of thinking for someone who has been suicidal for a long time (PSI).