What made Helen realize that nature wouldn't always be so kind?

The man realized this because of the sudden change of weather and thunderstorm.

In the novel If I Have Three Days of Light, Miss Sullivan asks Helen to sit in a tree and wait for her to go back for lunch. Suddenly, the weather changed and a storm came. At Helen's most critical moment, the teacher came to help her return to the ground. This incident made Helen understand that nature is not always friendly to people with a smile.

In the book "If Give Me Three Days of Light", Helen Keller warns the able-bodied people to cherish life and everything life gives them from the perspective of a weak woman with physical disability and firm will.