Dickinson was a big fan of nature. Maybe she didn't frolic around naked in it all day (as far as we know), but she did pack a ton of her poems with a ton of nature imagery. In this poem, she plucks images from the natural world and gives them human traits—a little poetic trick called personification. By doing this, she makes it crystal clear that she's not just talking about the life and death of flowers; she's talking about the life and death of humans too.
Line 2: The first whiff of nature imagery comes from the "happy Flower" of line 2. Notice that it's Flower with a capital F, which hints that it's not just any daisy in the field. No way, it's a symbol of some kind. You could debate about exactly what it symbolizes. Human life? Any life? Joy? Whatever it is, it's about to be taken away.
Line 3: Enter the villain. The Frost comes in carelessly lopping off flower heads. Once again, we've got a glaring symbol here. The frost = death—no doubt about that one. Check out how slick Emily is about working some personification in here with the word "beheads." She gives the frost a human trait by describing it as doing a human action: beheading someone else. (Kind of creepy that that's a human action, but that's the way it crumbles, cookie-wise.) It's not all about the frost, though. Emily also makes the flower seem human, too, by saying that it has a head to lop off in the first place.
Line 6: Line 6 zooms out and gives us an image of the Sun. It's cool how this line suddenly gives us a wider view. Everything up until now has been focused in on the plight of these tiny flowers, but now we're getting a broader perspective. It's interesting that the Sun is shown as so cold-hearted here. After all, it's the thing that gives the flower life. Why doesn't it care about the fact that the flower is dying from frostbite? Oh, because it's a sun, and suns don't care about anything because they're, you know, suns. In a way, this line de-personifies all the natural imagery in the poem, reminding us that none of it is human (and therefore not subject to human empathy). We might just be out of luck if we're looking for some kind of grand justification for the brutality of the world that would make sense to a human mind.