If the object of meditation were something concrete, something solid and graspable - an image or a statue or a dot on the floor or a candle - it would be much more of a concentration exercise. But the breath is very elusive; even if you wanted to give it one hundred percent attention, it would be difficult because it is so ephemeral, so light, so airy and spacious. As the object of meditation, it brings a sense of softness and gentleness.
Pema Chödrön, The Wisdom of No Escape: How to Love Yourself and Your World
meditation