The mood behind this poem is regret we notice how the narrator seems sad using a line like "And sorry I could not travel both," we see that maybe they had to make a hard decision that they regret. The undergrowth could represent the problems that are coming up under the surface and undergrowth describes a thick growing of plants under the trees so think of it as problems in a relationship that grow over time but are hidden by the bigger things happening. -Your friend, Bill Cipher
