Aliens are also known as extraterrestrial life. Their existence is questionable. However I strongly believe of their existence in such a huge universe out there. There are many other planetary systems similar to that of our solar system without doubt. As long as there is a habitable condition or environment on that planet, it could support life and without doubt will lead to the birth of aliens. Planet "Gliese 581 c" is considered a habitable planet due to its existence of water and having enough exposure to sunlight, around 30% of that of Earth, and large amount of greenhouse gases that warms up the planet to a point that will ensure the existence of liquid water.
This proofs that life similar to earth, or even more advance is possible on other planets. There could be a more powerful race on another planet, where they have achieved space age. Such alien race will begin exploring habitable planets out of curiosity, just like what we have been attempting to do. This will probably explains the reason behind alien sightings or alien's abduction.
When an alien race is dying or faces crisis due to their lack of resources or calamity ahead of them like 2012, and could not find any other solution besides travelling to their closest habitable neighbour, like earth. Alien invasion may occur. Even if it is merely to migrate from disaster, they might still invade earth. This is due to the human population, combined with alien population, will definitely break down earth due to over population. Also earth have not entered its prime in space travelling or anything, while aliens have already entered their golden age of space technology, this will make them capable of overwhelming our military easily. Why will they share earth when they can easily take it away from us. This basically led to an alien invasion. Such example of a game is Star Ocean series. It portrays armageddon, and alien invasion.

xx signed off at 11:06 PM