"I wandered lonely as a Cloud" is a poem that just makes you feel good about life. It says that even when you are by yourself and lonely and missing your friends, you can use your imagination to fine new friends in the world around you. As John Milton famously wrote, "The mind is its own place, and in itself, can make heaven of Hell, and a hell of Heaven." The speaker of this poem makes a heaven out of a windy day and a bunch of daffodils. His happiness does not last forever – he’s not that unrealistic – but the daffodils give him a little boost of joy whenever he needs it, like recharging his batteries.
Wordsworth would have the reader believe that the pleasure of seeing the daffodils with his "inner eye" is of the same quality and intensity (or better) than the real, momentary perception of them.
The speaker can only be happy in solitude by using his imagination. Therefore, he was lonely at the beginning of the poem because he was not using his imagination.