Aliens are an extraordinary claim, we need to check out a lot of things before we start considering them. Take the "life on Venus" recent claim. There was a lot of work done to even consider it. They used results from several observatories, analyzed every known explanations to rule them out one after the other, and even now they still consider it unlikely compared to other currently unknown chemistry. So if you see a scientist and tell him, I saw a flying saucer, it was aliens, he will laugh at you because there are thousands of other explanations to look for first.

On the other hand "mass hysteria" is also a lousy explanation. In reality, I think they are usually optical illusions. For example, the moon may appear like something else in unusual atmospheric conditions, and if you cross-reference the position of the moon with some UFO sightings, you get a lot of correlations. Other cases include small, close objects like insects appear like large, fast objects, this is common among pilots who are trained to look for other planes. There are also many psychological effects that aren't hysteria, for example it is common for people to "remember" things that never happened, which is also a big problem in a court of law: our memories are unreliable. Add to that defective equipment, pranks, etc... and you can probably cover most of the sightings.

Then there is a small minority of true mass hysteria, secret programs, unexplained physical phenomena, etc... and only then you can start to consider aliens.

And to make thing worse, more often than not, there is very little data.