I believe it, because look at it this way, the universe is MILLIONS of light-years big millions and millions of light years of space out there that makes earth look like a grain of sand.
It seems truly naive to say that out of a place so vast, we are the only ones with life. It's absurd. The conditions for life are evident in many places in space, and even if some planets only have plant life, that's how it started here on earth anyways, and now look at us.