Almost every line here turns an expected feeling upside down. Land, which we usually think of as safe and steady, troubles the speaker. The sea, which people often romanticize, is something they are "against." A ship makes them cry out, and bullets whistling overhead bring not fear but "a transport of joy." Even language slips: familiar words contain "unknown letters," as if the ordinary has started to look foreign.
The speaker seems drawn to whatever is strange, risky, or unreasonable. They trust a "thick-skulled fool" over sensible people, and they end by saying everyday business should be feared rather than served. The mood is restless and defiant, with a note of playful mischief, though the joy at danger also feels a little unsettling. The poem suggests a mind that finds routine suffocating and would rather be startled awake, even at some cost. The title, "On the Contrary," works as a quiet motto for the whole thing.