If you had the capacity to derive a lot of subjective, even ephemeral pleasure from your life by going on "adventures" and doing "awesome" things, you probably wouldn't be contemplating suicide in the first place.
That seems intuitively obvious and I've never understood why people think stuff like this is useful advice - nobody ever killed himself because he forgot to do all the stuff that that would've otherwise made living enjoyable.
I was about to harp on this point.
Like you said, suicidals don't have properly functioning emotions in the first place. They're not all that stereotypical "oh i'm sad i'm going lay around all day" types, but they don't enjoy life to begin with, are often very sad, very low energy, maybe constantly anxious. Not in an emotional state suitable to volition or one where they would expect (though it may not be true)
At the same time, you said you don't understand why people think it's helpful, and I think the intention is more to talk the person down. Try to cheer them up. It's not going to work, of course, but it's usually someone that has no experience feeling that way and so their understanding of it is a little shallow. Same as dipshits that think a depressed person can just will themselves to be happy. (Or dipshits that believe anybody can will themselves into any emotional state.)
SECULAR REASON NOT TO KILL YOURSELF
Suicide is extremely cruel to inflict on friends and family; regardless of the bullshit that suicidals talk themselves into, most people's family and friends (and even other people you may not consider friends but who are in your life) are deeply effected, often traumatized. If you know anyone that has suffered a sudden death in their circle like that you know how much it fucks them up. (Unless it's happened to you and that's why you're suicidal, in which case I'll give you a pass on that part.) That is an incredible cruel thing to casually force on someone else for causes that are
almost always temporary.
Suicide is like throwing a grenade into a big social web far bigger than yourself. You end your own suffering but you multiply it, several times over, in all the people you harm, prick.
Also, you don't want to be a pussy bitch, do you?
RELIGIOUS/SPIRITUAL REASONS NOT TO KILL YOURSELF
You were created with reason for the (primary) purpose of cultivating virtue and flourishing, which is exercised in large part by serving others (which I'd include nature as part of). Most people, myself included, are terrible at this. Struggle and turmoil is a necessary part of this, including grief,
the most intense form of suffering a person can know, as it involves a sort of miniaturized experience of death.
Your life does not belong to you, at least in that sense that people normally think. Your right to life is inalienable. You cannot give it away or surrender it because it goes against the very nature of what you were created for and your true nature as a human being. It is an offense to God and the nature of creation itself to engage in a selfish act of self-obliteration, and in its finality it closes off any hope of reconciliation. I am not convinced that suicides go to Hell, in fact I have a hard time believing that. But I can absolutely believe that it closes off some better, ideal state.
The only grounds on which you can possibly defend suicide are in situations where there is a reasonably high probability that death is the ultimate outcome, like a suicide attack in war, suicide in defiance, mercy-killing and self-mercy-killing of a terminally injured/sick person, and so on. Note that most of these are of a military nature and/or are foregone conclusions that the suicide merely fulfills.
