Why do you think the poet uses an image of a cloud to describe his own loneliness in "I Wandered Lonely as a Cloud"?

In the opening line of this poem, the speaker notes that

I wandered lonely as a cloud.

The image of a cloud serves a few purposes here. First, it immediately connects the speaker with nature, and that was a foundational effort of the Romantic poets. He will later contrast the almost-blank canvas of a cloud with bright and colorful daffodils; it is these daffodils which bring light and joy to his "cloudy" day.
Second, the simile provides some imagery to explain how he is moving. Clouds have no will of their own. They follow the trajectory laid out for them by air currents and winds. Likewise, the speaker is wandering in someone else's trajectory, not following his own path.
And third, the cloud is part of nature but exists apart from the earth's surface. It brings the much-needed rain that sustains life, but it does not directly interact with the abundance of life on earth. If a cloud had emotions, it would likely be a lonely existence: to observe life but not experience it. Thus, the author feels this same sense of loneliness, which he is rescued from in the final image by recalling images of the daffodils, which he spies along this journey.