This is a poem that clearly is about nostalgia. The way in which the west wind in the first stanza is described obviously indicates that the west wind brings with it a whole load of memories to the speaker of times long gone that he is now distant from both geographically and through the passing of time. Consider how the poem opens:
It's a warm wind, the west wind, full of birds' cries;
I never hear the west wind but tears are in my eyes.
For it comes from the west lands, the old brown hills.
And April's in the west wind, and daffodils.
The way in which the west wind results in the speaker crying at the thought of "the old brown hills" establishes this as a poem concerning the idealisation of the past. Consider the rosy-hued way in which the speaker's home is described:
It's a fine land, the west land, for hearts as tired as mine,
Apple orchards blossom there, and the air's like wine.
There is cool green grass there, where men may lie at rest,
And the thrushes are in song there, fluting from the nest.
Unrealistic descriptions and the complete absence of any negative descriptions clearly indicate the way in which we all often regard our past as being better and more trouble-free than the present. The "air's like wine" as if breathing it in too deeply would end up in the speaker becoming drunk, and there is rest in the "cool green grass" and the absence of labour and toil. Clearly this is a fictional, non-realistic memory of what the speaker's home was like, but this is a fiction that is not disturbed during the rest of the poem.