Newton’s Third Law of motion says that for every action is and equal and opposite reaction. One could say this about music, also. The major chord produces a sound that can be considered “happy” while the minor chord sounds “sad”. The interaction between major and minor chords is what drives many songs.
The Am, Em, and Dm are the most commonly used shapes, because of their ease of use. The Cm and Gm chord shapes are not as easy to play and therefore are not used as often.