If you were to encounter a crow out in the wild, it would be considered irrational to believe that it is the only crow out there in existence. The same goes as you scale up the chain, if you encounter one human in the wild, the odds are high that there is another human out there somewhere. The evidence that intelligent life exists out there is the fact that humanity exists.

I think it would be more arrogant to assume that we're somehow special or unique in what is an incredibly massive universe where we exist as mote of dust upon a mote of dust. Right now, humanity is in the stage of tossing messages in a bottle into an unimaginably vast ocean, while using a hastily built telescope attempting to see over the horizon.

This isn't to discount the incredible efforts of those people exploring the cosmos, but rather that to demonstrate just what we're working with.