I do believe in aliens, but then again, I don't. Let me explain.

To believe in aliens, you have to believe that there is at least one other planet capable of fostering intelligent life. I don't dismiss this possibility, so to that extent, I believe in aliens.

As for why I don't, it is a matter of time. Earth is a little over 4.5 BILLION years old. Homo Sapiens have only been around for about 200,000 years. We've only achieved behavioral modernity 50,000 years ago. We've only developed radio communication within the last 130 years. We left the planet about 5 decades ago. We have yet to get a human on another planet. 4.5 billion years and the most advanced species on this planet has never set foot on another planet. I don't know what the future holds but if the human species is in any kind of decline, our window of opportunity is closing real fast.

For all the planets that are in the habitable zones of their respective stars, it is possible that some of these will eventually create life. However, if the timeline of that occurrence is similar to the timeline here on Earth, then each one of those planets, ours included, will only see intelligent life capable of space travel for just a blink of time in a span of billions of years. I don't think intelligent life is created at the same speed or that the planets themselves were created at the same time.

So for us to see aliens or them to see us, our planets have to have their opportunities for intelligent life capable of interstellar communication or travel to exist at the same time. I don't see that being possible. Mars may very well get intelligent life someday through no influence from us. Will it happen during the period of our existence? I doubt it.