A poet could not but be gay,In such a jocund company: (lines 15-16)
Glee? Gay? Jocund? It’s as if he’s running out of words for "happy." The point is that the joy of the daffodils is irrepressible and contagious.
Which is the bliss of solitude;And then my heart with pleasure fills,And dances with the daffodils. (22-24)
No, wait, we were wrong. He has more synonyms for "happiness" in his bag of tricks: now it’s "bliss" and "pleasure". Notice that "solitude" is exactly what made him unhappy, or "lonely," at the beginning of the poem. Or maybe "lonely" just meant "alone," and we were not supposed to give it such a negative connotation.