It varies for me, yearly, monthly, daily.
Used to be all and only a matter of guilt ("What if there isn't a hell? What penance do I serve for my past existence and harms other than continuing to live and try to do better?")
Lately it's more the risk of letting people down when I've made commitments to help them, which is guilt adjacent I guess. (This is another reason that, when I'm pulling away from people, I'm aware that I'm at doubled risk, since it's not just isolation but losing that chance to be helpful).
And today, it's because of both (a) mere spite, and (b) an unsolicited compliment I received related to something important to me. Yesterday it was because I had the option to sleep away most of the day, and that was easier than doing anything more active to or with myself. Meanwhile, other days, there is no reason. But I don't always need a reason to live, to live.
Whatever reasons you're looking for right now, I hope they find you in the ways you need!