Do we matter?
In the grand scheme of things, nothing matters. In Universe time, hell, in Earth time, human existence is like blinking. There one second, gone the next. What we do, who we become, how we live, how we die, who we affect and how, are, in the grand scheme of things, meaningless.
Does this mean we are meaningless? I don't know.
If what we do doesn't matter in the big picture, how does it matter? To ourselves. We matter to ourselves. What we do, who we become, who we affect and how, etc, matters to us, our own personal selves.
In all honesty, if what we do affects nothing grand, nothing important, what's stopping us from being who we went to be? We can be the most selfish sons of bitches to ever live and it still wouldn't matter.
But then, maybe you don't want to be 'that guy.' Maybe you want to be the one everyone trusts and loves. Or maybe you want to be left alone. None of it matters, so it's all possible.
So the question isn't 'do we matter?', it's 'who do you want to be?'