Hubris, bravado and ego most likely. There is no evidence of any sort that would indicate this so this thought has to have manifested itself from some fantasy.
Actually I think you got that backwards. It’s not fantasy, considering that the only evidence that we have that life exists at all are ourselves and what we have on this planet. Of course it’s logical to assume that life exists elsewhere because the math and universe’s scale is staggering and it’s reasonable to assume that if there is life elsewhere some could be more advanced than us. However, until we find someone or something else, all the evidence we have is on this blue marble of ours…so if we are only going to believe what the evidence currently tells us…it’s that we are the only or first emergence of life.
All the rest—conjecture and faith. In essence other civilizations are the fantasy until more evidence is provided to the contrary.