actually no one has ever said this to me so here is the long answer!
Asana is a pose, which is to say, a place of repose. Not a place to rush through, as it sometimes seems in fitness classes.
When you learn the steadiness of mind and body to hold a posture for five breaths and more, and the basic alignment comes naturally to you, you will feel stable in that pose and not find it challenging in the same way you used to when it was new.
The novice mistakes this feeling of comfort for the attainment of mastery.
One solution is to continue learning new poses; there are thousands. When a posture is new, the mind is easily, and delightedly, attentive to the novelty and resulting difficulty. Coming into a new place with the same body is a thrill, not only because there is a new relationship between yourself and the world you can take for granted, but also because you are inspired to see what you can do.
But if this is all that motivates you, then you will find the extra-challenging postures to be depressing, and a deterrent. You will keep seeking out the postures you like, then feel that they are easy, and that you are good at yoga and need to try something else.
That is certainly one way to approach the practice, and life in general.
Another way is to be inspired by the particularly difficult, not for the emotional highs and lows of perceived success and failure, but for the simple work of being in the posture itself. The mind needs guidance to pay attention to what it thinks it knows— the breath.
In challenging postures, while we struggle for balance and alignment, the breath is erratic, shallow, forgotten because the mind is attentive to