This is mostly due to our natural cultural conditioning. Music written in minor key can sound happy, and music written in major key can sound sad. However, we are widely used to listen to sad music in minor key and happy music in major key.
Over the last years, it would seem that music isn't relying as much as it used to on major key for happy songs.
That being said, tonics in minor keys are more likely to appeal to us as they are farther away to everyday life's sounds than major keys.
