Why is the poet entranced by the butterfly?

The speaker in this poem is so entranced by the butterfly that he has been watching it "a full half-hour" when the poem opens. He seems, at first, to be particularly intrigued by the mysteries of the butterfly, repeating the word "motionless!" in an exclamation, curious as to whether the still creature sleeps or feeds.
The poet addresses the butterfly directly in this poem, exhorting it to "stay near me." He describes the butterfly as a "historian of my infancy," suggesting that it reminds him of summer days when he, as a child, saw similar butterflies. In the final stanza of the poem, the speaker confirms this to be true: he repeats the word "pleasant" to emphasize his feelings about earlier days with his sister Emmeline, when the two had chased butterflies. As a child, the poet was a "hunter" in his pursuit of the butterfly, which had enthralled him even then, although his sister was less boisterous in her approach to the creature.
The poet in the modern day, then, is entranced by this creature for dual reasons. On the one hand, he is reminded by the butterfly of happier days with his sister in his youth and how he felt compelled to follow the butterfly then. He is also intrigued by the butterfly because, as an adult, he is curious as to all the things he does not understand about it, such as how, or whether, it feeds or sleeps, and where it goes when it flies out of sight.