Answer:
The author sighs because he regrets that he had to choose one of the paths in front of him, leaving unknown forever, all the opportunities that the other path could present.
Step-by-step explanation:
The lines presented in the question above belong to the poem "The Road Not Taken" written by Robert Frost. In this poem, the speaker recalls a moment in his youth, where, while walking through the forest, he found himself at a crossroads where he had to choose one of the two paths to follow. Upon presenting the path he has chosen the speaker gives a sigh. This sigh reveals the speaker's sadness at not being able to use both paths, for when he chose the path less traveled, he abandoned forever the possibilities that the other path could offer and this makes him regretful.
These paths are used by the poet to make a metaphor about life, because when we make a decision, we ignore the results that another decision could offer us.