I used to think the same question by myself, and it came to me the aliens are too far away to exist. If the technology is advanced enough to find another planet just as the Earth and the Sun, it may has similar living things, we may call them--aliens.
There is no real concrete proof that aliens exist. But, I hope aliens do exist because that would mean we are not alone in this vast and unknown universe of ours. Someday we may discover them or they discover us.
A couple of responses would fit here, but, I'll stick with the one I think you are asking. I personally have never seen one. I don't count the movies. I would be pretty vain if I thought that the earth is the only planet that sustains life. What I do believe is yes, there is some form of life out there. We may not be able to perceive another being with our 5 given senses. Meaning, I'm not sure if I would be able to: See them. Touch them. Smell them. Hear them. or Taste them. I'm pretty sure they are out there. And if they are.... and they would come to visit,... I would invite them in to our house, have them kick back, put their feet up, grab a beer, and watch the lights of the city. I would of course ask, so how was the trip? Any Air turbulence on entry? Have a nice night!
Jill Tartar from SETI made a very good comparison when talking about exterrestial life in the Uuniverse. She stood at the edge of the ocean and dipped a cup of ocean water into a container. She said do you see any life? Just because life from the cup is not visible does not mean there is no life elsewhere !