I gazed--and gazed--but little thoughtWhat wealth the show to me had brought: (lines 17-18)
The speaker watches the flowers for a long time. He must be hypnotized. Interestingly, although he appreciates the scene at the time, he needs time to process and reflect on nature in order for its full value to become clear to him. It’s not a sudden revelation or epiphany.
For oft, when on my couch I lieIn vacant or in pensive mood,They flash upon that inward eye (lines 19-21)
Nature by itself does not create the feeling of deep joy and peace. It merely provides the raw materials. Nature must be worked on by the active imagination, the "inward eye." Note that Wordsworth does not claim that people can be transformed by nature all the time. They have day-to-day concerns that keep them indoors much of the time. But the daffodils are like that screen saver you have on your computer of some warm sandy beach in the Bahamas, reminding you that good stuff is out there in the natural world.