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 stabilizing the body. With time, steady body and steady practice, we learn to steady and then deepen the breath in each posture. The mind then gets lost, either in time (past/present) or place (going to work while you are on the mat!). Then the game is to use the posture as a place of repose for the mind: settling the mind into the here, the now.
This process is unique for each person and each pose. We develop relationships with each posture, dynamic in nature. A place of action becomes a place of attention and then a place of action, as the posture reveals deeper and deeper layers of the body-mind we can attend and elevate to a higher potential.
In the pose, we work on proprioception (where am I in space?) and interception (where is the space in me). The mind is attending not to novelty/difficulty, but in dynamic stillness, creating space for breath, and watching breath create new space in the body. These spaces are undoings, releases of stiffnesses and holding patterns that are almost never just physical— they usually reflect emotions, habits, and other layers of the psyche that are usually invisible to us.
Being in physical relationship between mind and body, we can apply energy to lessen chaos and create order. We reconcile paradoxes (strength/space; flex/extend; system/parts; gravity/levity; east/west), and create harmony where there was once conflict. This pairing of body, breath, and mind, invites a natural drawing inward of the senses: this is why asana, then pranayama, lead into pratyahara.
Working this way, deeper rather than collecting more and more new postures, is like traveling away from the shoreline where everyone is playing on the sand, covering distance along the edge of the waters while you go deeper and deeper into the ocean itself. There is magic there, along the ocean floors. Colors and experiences that you have maybe heard of and could maybe tell stories about but are most meaningful when you are living them.