The poem's title, "Positive thinking," sounds cheerful, but the lines underneath it are uneasy. The speakers, a "we," are busy and broke, yet they refuse to call what they feel misery. They also insist they can't be undone. It reads like a kind of self-reassurance, a story people tell themselves to keep going when money is tight and time is short.
The second stanza turns that comfort upside down. The "very bottom of every thought" is described as ground, something solid we might stand on, but we "ruin" it by "putting in a mine." The image suggests that our own optimism or denial digs away at the foundation we depend on. A mine can also be a trap, a buried explosive, which adds a hint of danger.
The mood is quiet, dry, and a little ominous. The speaker doesn't shout a warning. Judging by the imagery, the poem suggests that positive thinking may be less a strength than a way of overlooking what is slowly wearing us down.