Basically, when you start looking at possible, likely, and popular theories of the apparent existence of "UFOs" then you find three quite different things. But our opinions on what is likely or popular are quite different also, so all these differences are part of what makes the topic of "UFOs" so controversial.
Possible explanations of UFO existance cover the entire spectrum, from the alien theory to my time travel theory, to whatever reason is given in popular science fiction TV shows and movies. They all are possible, accepting that everything and anything is possible, which is an understood concept. If you think that the idea of UFOs being ran by giant space monkeys is impossible, than so are infinite other possibilities, so simplification is achieved by making the assumption that anything is possible.