What does the speaker in "I Wandered Lonely as a Cloud" see that changes his mood? How?

In "I Wandered Lonely as a Cloud" by William Wordsworth, the speaker is out walking in nature. His mood is "lonely" as he wanders aimlessly along. Suddenly, he sees a field of daffodils. One imagines him coming up over a rise and seeing spread out before him a luxurious field of the yellow flowers blowing in the breeze. Stanza 3 describes the change in mood the speaker experiences upon seeing the unexpected flora. The flowers, he imagines, are more gleeful than the dancing waves. He catches the spirit and says, "A poet could not but be gay, In such a jocund company." Jocund, not a word we use often, means lighthearted and cheerful. Gay is a synonym. The flowers seem happy because of their bright yellow color and their free and easy movement that looks like dancing. Their imagined happiness is catching; the poet takes on a bright and joyful mood just from watching the field of flowers.
Another mood change is mentioned in the last stanza. Now the poet feels either "vacant" or "pensive." Vacant means that nothing important is occupying his mind. Pensive means that he is thinking of weighty or serious matters. At such times, the memory of the field of daffodils comes back to him, and again it changes his mood, causing him to feel just as cheerful and lighthearted as when he first came across the golden field of flowers.