I was asked recently if I believe in aliens.  Here's what my response would have been if I wasn't drinking: 

1.  I fundamentally believe there's a natural explanation for everything.  This is called naturalism, and it's the opposite of supernaturalism (e.g. religion).  Human beings have, for a long time, come up with supernatural and sometimes absurd explanations for things they didn't understand.  "God created the earth, then the sun" (oops, wrong order).  "Earthquakes are god's punishment for something" (oops, plate tectonics).  "You're sick because of bad air" (oops, germs). 

That's not to say people are big dumb stupid idiots.  I would go with less aggressive words like "ignorant" (i.e. didn't know) or "naive" (i.e. didn't know enough).  As we learned more, there was less of a need for supernatural explanations for things because we discovered natural explanations.  There are literally countless examples throughout the history of scientific discovery. 

I believe aliens fit in that category.  The pictures, videos, and eyewitness testimony all follow the pattern of, "I don't know what I'm looking at, so it must be aliens."  It's the same with "ancient aliens" who supposedly helped us build the pyramids and whatnot.  "I can't imagine a way humans could've built this structure, therefore aliens did it."  It's a failure of imagination. 

I don't have a good answer or explanation for things people attribute to aliens.  And I understand the thought process of appealing to that logic, because it does make a certain amount of sense.  But based on our well-documented human history of mis-attribution, I'm holding off judgment until we get some better data. 

2.  However, I also think the existence of aliens is pretty likely.  Another common theme in human history is thinking we're the center of everything and assuming we're unique.  We used to think the earth was the center of the solar system and the center of the universe, but it turned out to be sort of the opposite.  We used to think biological life was temperamental and rare, requiring just the right mixture of air and water and sunlight, but then we found things on hydrothermal vents at the bottom of the ocean.  Life is not difficult, and we're not unique. 

If there are [whatever] billions of galaxies in the universe, and [whatever] billions of stars in each galaxy, the idea of a rocky planet in a goldilocks zone hospitable to some sort of life is at the very least conceivable, if not a near certainty.  It would be the ultimate example of human hubris to assume we're alone in the universe.  It might be alien bugs or alien lobsters instead of alien humanoids, but still. 

3.  That being said, I think the pro-alien lobby (just made that up) is a little short on evidence.  Claiming a certain rock couldn't have been cut or moved by people of a certain era because they didn't have the proper tools, and then claiming it's aliens -- this is like answering a question with a question.  You couldn't come up with a reasonable explanation, so you went with unreasonable.  Or capturing a shaky-cam video of some lights in the sky that look weird -- like seriously have you never heard of image stabilization?  It's like Bigfoot sightings:  You're telling me in the 200 or so years of the existence of cameras, we haven't been able to capture a single clear shot? 

Discounting government cover-up conspiracy theories (a topic for another day), the evidence has yet to be compelling.  Interesting?  Yes.  But conclusive?  No.  However, to quote Bill Nye when he was asked what if anything would change his mind:  "Evidence." #science