Lines 13-14

The waves beside them danced; but they
Out-did the sparkling waves in glee:

• The waves also dance in the breeze, but the daffodils seem happier than the waves. We know from Dorothy Wordsworth’s journal (see "In a Nutshell") that the day that inspired this poem was a stormy one, so the waves on this medium-to-large sized lake must have been larger than usual. Maybe they were even cresting into whitecaps.
• The point is that the entire scene has suddenly been invested with a joyful human-like presence. Since waves do not bring as much joy as the yellow flowers, the flowers "out-did" the water with their happiness.
• The waves "sparkle," which creates yet another association with the stars. Everything seems to be gleaming and twinkling and shining and sparkling.

Lines 15-16

A Poet could not but be gay,
In such a jocund company:

• The speaker reenters the poem. (We’ve haven’t seen you since the first line, buddy.) Except he refers to himself in the first person, by his vocation, "a poet."
• Despite his earlier loneliness, the speaker now can’t help but feel happy, or "gay," with such a beautiful vision to look at.
• Or, as he puts at, with such joyful and carefree ("jocund") "company" to hang out with. The flowers and waves feel like companions to him. They are all pals. Group hug!

Lines 17-18

I gazed--and gazed--but little thought
What wealth the show to me had brought:

• The repetition of "gaze" tells us that he kept looking at the flowers for a long time. It's as if the speaker enjoys looking at these daffodils at the time, but doesn’t realize exactly how great of a gift he has just received with this vision.
• Apparently, the speaker doesn't think that he fully appreciated the vision at the time. This is a bit odd, because he seems to be really enjoying those daffodils.
• The word "wealth" expresses a more permanent kind of happiness. It also carries a hint of money that does not quite fit with the supernatural language that has come before.