from a list of zen koans found here:https://www.huffingtonpost.ca/entry/zen-buddhism-koan_us_563251dce4b0631799115f3c
When asked why he practiced zen, the student said, “Because I intend to become a Buddha.”His teacher picked up a brick and started polishing it. The student asked “What are you doing?” The teacher replied, “I am trying to make a mirror.”“How can you make a mirror by polishing a brick?”“How can you become Buddha by doing zazen? If you understand sitting Zen, you will know that Zen is not about sitting or lying down. If you want to learn sitting Buddha, know that sitting Buddha is without any fixed form. Do not use discrimination in the non-abiding dharma. If you practice sitting as Buddha, you must kill Buddha. If you are attached to the sitting form, you are not yet mastering the essential principle.”The student heard this admonition and felt as if he had tasted sweet nectar.
I think this also has to do with advising not to attach yourself to any experience in practice,like tasting sweet nectar.I think another part has to do with ego (this student trying to be Buddha).Trying to be something less than what we already are.We should instead focus on moving to that self realization that we are everything when I means nothing.