Gravity is definitely not constant. The moon & planets have an effect (as tidal changes prove). I specifically remember hearing that all the planets were in some sort of rare super alignment when the quakes happened that caused the Japanese or Indonesian tsunami. Makes sense to me that the change in gravitational field across the entire solar system essentially popped the earth's back. So, while I'm not sure that really has a significant impact on people's personalities or their daily moods, it's definitely not nothing.