My friend Kevin Kelly, a co-founder of Wired and my partner at Recomendo, says, "I have some strongly-stated, loosely-held hunches about the probability of life elsewhere in the universe. My certainty about these beliefs is quite low, but it is much higher than my belief in the alternatives."
You can read all 12 assumptions here, but here are the 10) and 11):
10) The only reason for an advanced civilization to visit another planet is to see if there is another civilization which has invented things it has not, and perhaps could not invent. Invented resources are thus unlimited in scale and scope, and can be discovered only in unique places in the cosmos. Interstellar travel is essentially not travel through cosmic space but travel through possibility space. You visit another planet to visit other possible minds to see if they have thought of fabulous technologies your collective minds cannot reach.
11) Every day a few probes of these billions of interstellar civilizations visit our planet scoping out our technological state. These technological probes appear briefly in order to see us, and disappear once they have inspected our inventory. So far we have little to offer; nothing that can't be found on millions of other planets.