The speaker opens with an odd, tender apology: addressing doors, they say sorry for "having a house." It's a playful way of admitting that shelter means shutting something out. A house needs doors, and doors divide inside from outside. The title, "Insider," fits that idea. The speaker is on the safe side of the threshold and feels a little guilty about it.
The second stanza is knotty, and its grammar is deliberately strange. What seems clear is that the speaker calls this need for walls "a common evil," something ordinary and slightly shameful, required for "quiet or content in the rain." The last lines turn to the rain itself, which "tends to sing well / and eat nothing at all." Judging by the imagery, the rain is gentle and harmless, a music that takes nothing. The mood is hushed, wry, and contemplative: cozy comfort mixed with a faint unease about wanting it.