Personification is so prominent within "I Wandered Lonely as a Cloud" that it can almost be considered the poem's "mode." This device mainly functions by ascribing human emotions to the natural world. This is not a one-way exchange, however—it is part of an overall argument for a closer relationship between humanity and nature. In seeing elements of itself in nature, humanity can see itself as part of nature.
Three different aspects of nature are personified in the poem. The first personification occurs in the first line. It's subtle, because it's wrapped up in the speaker comparing themselves to a cloud—which is, in fact, the reverse of personification. But the speaker characterizes their "wandering" as "lonely," and specifically akin to the loneliness of "a cloud." In reality, loneliness is a human emotion and, of course, not one felt by clouds. This personification helps to link the human speaker with the natural environment in which they walk.
The main personification applies to the daffodils. They are described as a "crowd" and a "host" (the latter of which also has the subtle connotation of relating to angels), and on three occasions they are said to be "dancing." Dance is particularly important—people dance as an expression of joy, and/or as a method of togetherness. Dance has also played a key role in human history as a ritualistic expression of spirituality. As the poem is about joy, togetherness with nature, and a spiritual way of seeing humankind's place in nature, the projection of dancing movement onto the daffodils reinforces all of these ideas.
The third personification is in line 13, when the waves are also described as dancing. Here, there is a sense that the speaker is perceiving nature in motion, and though the daffodils "outdo" the waves in joyfulness, the overall effect is that the entire scene contains an essence fundamental to human happiness.