The answer to your question is easy; the reasons could take many pages to support.
The bottom line for the reasoning, though, is this: the "extremely pessimistic, soul destroying" person has a major investment (much of his identity, personal history, emotional habits, set of coping mechanisms, etc.) in his dark attitude, so, chances are there is not a damn thing you can say that is going to get him to turn his attitude and life around. People ignore or rationalize dramatic consequences of their own life decisions, so a few words, even magic words, will not make a person WANT to look for a better way to live. (In my life I've asked your question several times for various versions of the story. It took a while to get through my thick head, but I, uh, began to see a pattern . . . )
My suggestion for the advice you should give is simply this: "My friend, I think maybe your extremely pessimistic attitude is poisoning your own life. I am certain it is poisoning mine. Apparently you have mistaken my friendship as solely a source of sympathy instead of as acceptance of you while you work to build your happiness. So, I cannot insist that you seek professional help, but I can insist that you keep your soul-destroying negativity out of my life. Good luck, my friend, but good bye."
It sounds a bit cold, but I came to this after finally admitting to myself that yes, I can, in fact, generate more than enough pessimism and negativity for myself to handle without having not just my mood of the moment but my life being infected by others who have strongly confused self-love with self-sympathy. In other words, I am perfectly willing to get myself all kinds of dirty while trying to help someone crawl out of a hole, but I've learned to tell the difference between someone who is reaching out for help and someone who is reaching out so he can pull others down into the mud with him (because Misery DOES love company.)
Tell your friend (or family member!) you tried long and hard to help him, but you just can't tolerate his negativity, and then YOU do exactly what you want HIM to do: go out and surround yourself with positive energy.