Written in Germany
If I had but two little wings
And were a little feathery bird,
To you I'd fly, my dear!
But thoughts like these are idle things,
And I stay here.
But in my sleep to you I fly:
I'm always with you in my sleep!
The world is all one's own.
But then one wakes, and where am I?
All, all alone.
Sleep stays not, though a monarch bids:
So I love to wake ere break of day:
For though my sleep be gone,
Yet while 'tis dark, one shuts one's lids,
And still dreams on.
Written in Germany, presumably while far from his love, Coleridge dreams of his lover. The almost literal flight of fancy in the first stanza is something we've almost all felt at some time. "I wish I could go to you" is the basic idea, but sadly, none of us can turn into birds and fly to our lovers. Coleridge well knows this, hence those thoughts are "idle things" and yet he cannot help but thinking them.
Starting his second stanza with "but" Coleridge is acknowledging that yes, it is a childish sentiment (as indicated in the title) but regardless, the world of dreams is a magical place, where separated lovers can once again be together. Just as in the first stanza though, there is a snapback to reality. The narrator wakes from his dream, still alone.
Like so many of us do when waking from a pleasant dream, he closes his eyes once more, and pleasantly continues to dream of his lover. I wonder though, why Coleridge feels the need to identify the dream flight to one's lover as childish. Yes, it is impossible to take wing and speed away to one's beloved, but that is more an affliction of the adult condition than the childish one. I do not think he means to be dismissive by calling something childish, though. There seems to be an intrinsic wonder and value to these dreams, justified by both Coleridge's narrator's indulgence in this fantasy while he wakes, and also by Coleridge calling this phenomenon "very natural" in the title. There seems to be a real veneration of nature and of our propensity towards dreaming silly thoughts.