Poetry Slam!: "We Grow Accustomed to the Dark" by Emily Dickinson

Note the word "custom" ("accepted or habitual practice") within "accustomed". We usually get accustomed to something by deliberately practicing, using, or making a habit of it. But the dark is something that is all around us, that we cannot control, and that we don't actually want, yet we must grow accustomed to it, in order to grow in our lives.

Being a witness is being able to see, which one needs light to do. The neighbor holds the lamp so that she can see us to say goodbye. The image of light as a neighbor suggests that, unlike darkness, light is not a permanent part of our lives.

The chosen definition fits the example sentence because Dickinson is personifying Life as someone who is walking and taking steps. But "straight" also means "without evasion or compromise," "characterized by honesty and fairness," "in a forthright manner" and "following a correct or logical method"--all of which would fit when you see darkness as more than just a physical absence of light.