The speaker imagines other lives that might have been ours: drifting like albatrosses on the wind for years, or sitting around playing cards, giving darkness "a respectable home." In those lives we could have dipped into our own shadows and tasted salt. The tone is wistful, with a dry, almost wry humor in the idea of darkness being made comfortable and respectable.
But the poem turns on "instead." We chose the sun and spent our lives "digesting light for a living," a phrase that makes living in brightness sound like a job. The cost is that we avoid our shadows, the darker, quieter parts of ourselves, until death, when we finally lie down on top of them.
The ending is quietly unsettling. Our "vital bitters" seep out of us, as if everything we never faced is released only when it's too late. The mood is reflective and a little regretful, and the poem seems to suggest that ignoring the dark side of life leaves something unfinished.