Here's why:
* The title itself: "So We'll Go No More a-Roving" immediately sets a somber tone. It suggests an end to carefree wandering and a shift towards something more settled.
* The speaker's voice: The speaker expresses a weariness with the "roving" lifestyle, highlighting the sadness and loneliness that accompany it. He seems to be acknowledging that the youthful exuberance of exploration has faded.
* Themes of love and loss: The poem alludes to a lost love, adding another layer of sadness. The speaker's regret and acceptance of the situation contribute to the overall melancholic mood.
* The imagery: The poem uses imagery of nighttime and darkness, further emphasizing the poem's somber tone. The speaker's desire to stay indoors and "sleep" also reinforces the sense of weariness and resignation.
While there's a hint of acceptance in the poem's ending, the dominant mood remains one of melancholic acceptance of a life that is no longer filled with carefree exploration.