What is the reason one should be humble in all circumstances

Humility is a quality in which a person does not become arrogant when he achieves success.

Humble people are grounded.

Why should the winners also stay humble?

It is because, in the game of life, everyone loses.

There is no winner in the game of life.

Consider tragedies like the death of a loved one or the death of oneself, accidents, physical disabilities, ailments like cancer and diabetes, financial hardships, and so on.

One would come to the conclusion that ultimately life defeats all. There is not even a single winner simply because no one has conquered death.

In the Bhagavad Gita, it is stated that death is the greatest truth of life.

It could also have been stated that defeat is the greatest truth of life.

These are the reasons that one should always be humble irrespective of temporary victories, no matter how glorious.