[Zanthor]

Another thing that seems to help is building a medical lab facility. They do three things. Stops up to level 3 plauges, helps population grow faster(1%) and helps the system population grow happier. Also I have been told to leave a fleet of 10 or more ships over the planet. Not sure if that works though.

It is important to know how you set the game up. You can be neutral, bloodthirsty, and peacful. It defaults to peaceful. They dislike attacking and invading. Bloodthirst is the opposite. If you fail in battle or are not at war with someone they aren't happy. Nuetral doesn't like contact or treaties.