Is there anything inherently wrong in believing in things that can't be proven to exist?

So what if someone believes in Star Wars, Buggs Bunny, or some god? As long as it doesn't lead to violence, what does it really matter if they believe in such things if it makes them feel better or gives their life meaning? It seems like it only becomes a problem when they try to push it on other people, which they do at times, but beyond that it seems relatively harmless.