To assume that we, as humans, are the only intelligent life (intelligent being defined as capable of developing cultures and technology, forming language and essentially building upon ourselves as a species) in our universe is perhaps as arrogant as it is ignorant. However, comparably ignorant is the common argument that there must be millions of other intelligent species, given the size of the universe and the number of planets just within a telescope's range of Earth. In the 1960s, Frank Drake formed an algorithm to calculate the probability of another intelligent species existing on a nearby planet. While this rather rudimentary formula is actually quite useless with our current knowledge, he was among the first to reasonably consider the likelihood rather than merely declaring that "there must be millions."
Our planet exists in a series of what are called Goldilocks Zones. Not too much, not too little, just right. We are in the Goldilocks Zone of our solar system. Much closer to the sun, and our seas would be vapor. Much farther from it, and they would be ice. The size of Earth also lends well to supporting life as do the speeds of orbit and rotation. If our Moon were larger or closer, the tides would become unruly, thus eliminating the possibility of life within several miles of any coast line.
Even our Sun's age as well as this solar system's position in the Milky Way galaxy lend well to the existence and development of intelligent life. Since the galaxy is slowly rotating, there exists a stronger pull of gravity towards the center, much like if one were to approach Earth's core. Beyond that, even our galaxy's position in the universe and the spacing of its stars exist in something of a Goldilocks Zone. Too much space between the stars, and the empty space grows too cold to support a life-hosting planet. Too little, and the vast nothing is too hot. Michio Kaku speaks more extensively on the science of these higher order Goldilocks Zones in his book "Physics of the Impossible."
So, other intelligent life in our universe is too probable to discount it as nonexistent. However, with the many factors and Goldilocks Zones to consider, it's nowhere near as likely as many a believer would quickly assume. This also means that it's unlikely that any of it is close enough for us to reach it any time this century, sadly enough. In fact, other intelligent life in our galaxy would need to be approximately as far from the center as we are, leaving only a narrow sliver of possible stars on this phalange of the Milky Way. If another species is able to reach us, they have little to gain from visiting our relatively primitive planet, short of subjective fascinations with a similar but lower species, like a planet-hopping Jane Goodall.
Really though, perhaps the best stated interpretation of our failure to find other intelligent life has come from none other than xkcd's Randall Munroe. Yes, a webcomic artist, oddly enough. "But seriously, there's loads of intelligent life. It's just not screaming constantly in all directions on the handful of frequencies we search."