We have to get rid of the usual debris; the usual qualifiers skeptibunkies give when talking about UFOs: yes, many UFO sightings are cases of misidentifications: stars,planets and other natural phenomena; military or otherwise high tech human made craft, or weather balloons, there are the hoaxers and the unstable and deluded who think that just about every mundane object is an extraterrestrial craft.
Still leaves us with a large body of genuine unidentified craft. Objects. Machines. Presumably with some kind of sentient being either inside of them, or at least responsible for their flying around.
Convoluted explanations, like insisting that all of the previously mentioned reasons are the reasons, and the only reasons, don't explain anything. It's just stubborn ignorance.
Some researchers suggest it's a sort of symbiotic dance between our psyche and "them."
I don't reject that entirely, but it's not the only explanation. If we've learned anything, there is no one fits all answer to the UFO question.
Some witnesses and experiencers take a literal, fundamental approach involving religions and dogma; interpretations and systems surrounding God, gods, Gods, Satan, Baby Jesus. (A look at many of the recent articles on UFO Digest will show you that.)
The government pretending that they're UFOs to cover up covert activity; much of it illegal, say, with UADs.(Unexplained Animal Deaths, as in cattle mutilations) is another common explanation. I don't discount that either for some of the events.
This is not to say some of the above can be actually true in some cases for some things. (For example, UADs. I'm of the opinion it's not aliens responsible for these cruel deaths, it's government/private corporations.)
But, while some of the above are likely in some cases, it doesn't explain it all.
What's left: ET. Actual beings from other planets, either within or without, our solar system. Or, from both within and without our solar system.
All the questions that arise out of this idea, like why would anyone want to come here, how could they travel such a long way, and so on, are usually spouted by the skeptoid to dismiss the idea aliens are here. Or, would even think to come here. Some like to throw in they don't doubt aliens exist, just not around here. More disingenuous dismissal.
Since no one has a clue what they're about, how can we presume to know why they'd do anything? We can only apply what we know about us, as humans. Why would we want to go and study "primitive" tribes? Why are we going out in space?
Add to this the fact that the body of evidence about flying saucers and beings throughout history, including prehistory, and there is only one conclusion: there is definitely a there out there. Or, here.
The simplest answer? They're here. They've been here for awhile. There's more than one kind. Different species, or variety, or races or whatever we want to call them, have different agendas, ranging from supreme indifference to interference, for good or bad.
And, some of them may or may not be, as I mentioned earlier, able to interact with us not just technologically, but on that spiritual/psychological level.
But simply put, aliens exist, flying saucers exist, and that's the end of that.
I realize I have no proof; no one does. Unless there really is a body somewhere hidden away in some ultra top secret bunker belonging to some government. Still, if they do, none of us know that, which means we can't prove it.
Really, all I have is opinion. A strong opinion, which doesn't make it fact. Do I dare come out and say "belief?" Okay, sure. I said it. I "believe" they're here, if that makes one feel better. As long as you and I both understand beliefs are capable of being changed. So don't get comfortable and think my belief is a dogma, an unshakable thing. It isn't. You can't use that to smugly dismiss me as some kind of nutcase. Won't work. You know: the old "beliefs in UFOs are just a new kind of religion to replace a spiritual void and emotional dissatisfaction with mainstream religion." There are those who do buy into this "belief" for those reasons, but I don't own that.(For one thing, I don't expect the aliens to save us from a damn thing, heal the planet, stop war and famine and poverty, or cure cancer.)
Between the things I've seen, the experiences I've had, and the reasons given here, I "believe" aliens exist. Seems like the most reasonable possibility.