its not as uncommon as one might think. a simple google search will lead you to a list of people with varying cases. i havent known someone personally like this though.
from an outside perspective, im a happy person, i have a decent life, and id have no reason to do this. and honestly, my life right now isnt bad, objectively speaking, but i have been through way too much shit before this. i've been abused in too many ways, too frequently, and when it stopped all i could think about is "i cant let it happen again", and the only way to be sure is by not existing anymore.
people may seem happy but we dont know their before and their inner thoughts. eberyone has a pain limit, and sometimes we reach ours and cant risk going beyond it