I am personally quite sure that there is no thing that gives meaning to our lives, there is no physical law that decides whether an action is good or bad and there is no purpose. Believing this seems rational to me, but why?
To start, what do "meaning" and "purpose" even mean? What is the meaning or absence of meaning? What are the arguments for and against any meaning or purpose in life?
Nihilism almost feels a bit like a religion to me. You just believe it or you don't, except that it is a bit more rational to believe. We can't prove that unicorns do not exist, but since there are no records of any, it is reasonable to believe that they don't.
The same applies to meaning. However, if some alien would claim that crocodiles exist (and let's assume crocodiles do not exist where the alien lives), then still the alien would be right, since they do exist here on earth. Maybe unicorns exist on another planet as well. In the set of all possible creatures some do exist.
So in the set of all possible meanings, some might exist as well. So how can Nihilists be so sure that there is no meaning? Not believing in a particular meaning does not imply that there is none. There must be some reason why we exist, not as humans but as matter.
Why do physical laws work in the way they do? There must be a reason. Not a magical one like a god, but a logical one. And this would defy nihilism.