What you and I do today is meaningless in the grand scale of the universe, and likely has a tiny effect on what happens to someone living a hundred years from now.
That doesn’t mean that what we do doesn’t have a more immediate impact.
Make your neighbor’s day better, because while it won’t matter in a million years, it matters now. So who cares if it costs you a few extra minutes of your life, it makes theirs better, and nothing means anything in the long run anyway, right? So why not make it easier for everyone else here, now? Making other people feel better feels good, so everyone wins, and we can better enjoy what time we have.