On first read, the last line of "Filling Station"—"Somebody loves us all"—seems to come out of nowhere. The majority of the poem's energy is spent on depicting the overall trashiness of the place. But after a closer look, you'll see the foundation for this last line had been building very quietly from early on in the poem. It begins when we find out it's a family-run station. And when the close of the poem starts up with the "Somebody" repetition, we see the care taken to make this nasty old place a home. What keeps it together (even if in a filthy state), it seems, is the love and care of the people living there.
The last line comes out of nowhere. It's the speaker's unrealistic fantasy that there's any love in this place. She should have stuck to the grease and grime rant that she was on in the first place.
The love (last) line has been built not explicitly upon showings of love in the poem, but small showings of care—watering the plant, decorating the doily—and it's a perfect last statement to end the poem on.