When you are balancing perfectly in a tree pose, everything is easy; your breath is deep and relaxed, and your muscles are working for you just as you'd like. It's pure and simple. Efficient. When you are having a great day, the same things occur. Your breathing is relaxed, your body is working harmoniously with your mind; everything just feels easier because you are in a state of balance.
Tara Stiles (2012). “Yoga Cures (Enhanced Edition): Simple Routines to Conquer More Than 50 Common Ailments and Live Pain-Free”, p.33, Potter/TenSpeed/Harmony