And Winter's dregs made desolateThe weakening eye of day. (3-4)
Golly. With descriptions like this, who wouldn't want to be outside? The use of the word "desolate" does a lot of work here to empty the poem's landscape of anything – human or otherwise.
And all mankind that haunted nighHad sought their household fires. (7-8)
The punch lines of the first two stanzas are reminders that 1) the natural world is foreboding, and 2) our speaker's all alone in it. We'd like to mock that never-ending gloom, but it's part of what makes the thrush's song so miraculous. Birds singing on a happy, sunshine-y day? Pretty normal. Birds singing in the dead of winter? Now that's something.
And every spirit upon the earthSeemed fervourless as I. (15-16)
Our speaker's totally rubbing it in now. No people. Lonely, lonely, lonely. Notice how this quote starts with the word "and"? It helps to create the sense of cumulative loneliness…as in, our speaker's going to point out all of the ways that he's alone. All of them.