When one door of happiness closes, another opens; but often we look so long at the closed door that we do not see the one which has been opened for us.

The image of closing and opening doors frames happiness as dynamic rather than fixed: opportunities and sources of joy change, and loss is not the end of possibility. The sharper point is psychological—grief, regret, or fixation can narrow perception so that new options go unnoticed. Keller suggests a practice of attention: acknowledge what has ended, but do not let it monopolize vision. The line is often read as encouragement to cultivate adaptability and hope, emphasizing that the obstacle is not only external circumstance (a “closed door”) but also the internal habit of dwelling on it.