The reason the Aliens keep coming to Earth has pretty much remained a mystery. Straker theorized that the Aliens might be a dying humanoid race which had become sterile, and are coming to Earth to extend their own lifespans using human organs for replacements. One person who seems to be speaking for the Aliens confirms that their planet is dying, and they mean no harm to the people of Earth.
However, there is some doubt as to whether the Aliens are humanoid at all, as the remains of an Alien body appears to be 100% human in origin. Dr. Jackson theorizes that the Aliens may not be humanoid at all, but are merely taking over the minds of humans and using the bodies as "vehicles". This theory is backed up as the Aliens appear to have taken over the mind of a cat.
However, the Alien's behavior sometimes remains confusing. Like when they planted bombs, or tried to wipe out all life on Earth. This is simply not consistent with their previous statements that they wish no harm to the people of Earth, nor is it consistent with a race that requires a large supply of healthy human bodies.