If presents being less thrill now that we are grown up, perhaps it is because we have too much already; or perhaps it is because we have lost the fullness of the joy of giving and with it the fullness of the joy of receiving. Children’s fears are poignant, their miseries are acute, but they do not look too forward nor too far backward. Their joys are clear and complete, because they have not yet learnt always too add ‘but’ to every proposition. Perhaps we are too cautious too anxious, too sceptical. Perhaps some of our cares would shrink if we thought less about them and entered with more single-minded enjoyment into the happiness that come our way.

The author of the passage is against

The correct option is "worrying too much about the past and future."

Let us break down the author's argument step-by-step to understand why this option is correct:
1. Analysis of Children's Happiness: The passage states that children's joys are "clear and complete" because they do not look "too forward nor too far backward." This means they live in the present and do not worry about the future (forward) or regret the past (backward).
2. Contrast with Adults: In contrast, the author suggests that adults are "too cautious, too anxious, too sceptical" and that "some of our cares would shrink if we thought less about them and entered with more single-minded enjoyment into the happiness that come our way."
3. The Author's Stance: By highlighting that anxiety, caution, and looking too far ahead or behind diminish our happiness, the author is arguing against this habit of worrying too much about the past and the future. Instead, the author advocates for single-minded enjoyment of the present moment.