Chogyam Trungpa again:

When you slouch, you are trying to hide your heart, protecting it by slumping over. But when you sit upright in the posture of meditation, your heart is naked. Your being is entirely exposed- to yourself, first of all, but to others as well. Through the practice of sitting still and following your breath as it goes out and disolves, you are connecting with your heart. By simply letting yourself be, as you are, you develop genuine sympathy toward yourself. When you sit erect, you proclaim to yourself and the the rest of the world that you are going to be a warrior, a fully human being.