Learn English – How does a dash end the sentence? Is the sentence supposed to be loose

A dash used like this signifies a sudden change of mood.

Imagine that the narrator (a dog) is reading her story and breaks off one train of thought abruptly. In this case, the narrator is reminiscing fondly of her puppy, and then remembers the next, less happy, part of the story:

By and by came my little puppy, and then my cup was full, my happiness was perfect. It was the dearest little waddling thing, and so smooth and soft and velvety, and had such cunning little awkward paws, and such affectionate eyes, and such a sweet and innocent face; and it made me so proud to see how the children and their mother adored it, and fondled it, and exclaimed over every little wonderful thing it did. It did seem to me that life was just too lovely to—

Then came the winter. One day I was standing a watch in the nursery. That is to say, I was asleep on the bed. The baby was asleep in the crib, which was alongside the bed, on the side next the fireplace. It was the kind of crib that has a lofty tent over it made of gauzy stuff that you can see through. The nurse was out, and we two sleepers were alone. A spark from the wood-fire was shot out, and it lit on the slope of the tent. I suppose a quiet interval followed, then a scream from the baby awoke me, and there was that tent flaming up toward the ceiling! Before I could think, I sprang to the floor in my fright, and in a second was half-way to the door; but in the next half-second my mother's farewell was sounding in my ears, and I was back on the bed again., I reached my head through the flames and dragged the baby out by the waist-band, and tugged it along, and we fell to the floor together in a cloud of smoke; I snatched a new hold, and dragged the screaming little creature along and out at the door and around the bend of the hall, and was still tugging away, all excited and happy and proud, when the master's voice shouted: "Begone you cursed beast!" and I jumped to save myself; ...