Sarah is compared to snow: lovely to look at, but cold and hard to approach. The poem's central idea is that snow doesn't stay that way. Hold it long enough and it changes, and the speaker says the same is true of her. Her guarded, "merciless" exterior isn't who she really is, just a wall that can come down with time.
The "he" in the poem understood this. Instead of pushing or giving up, he waited, and eventually she melted. The tone is quiet and a little sure of itself, almost like a small story with a satisfying ending. The last lines, "it was all because of / him," give the poem a slightly proud edge. Whether that feels tender or a bit possessive is up to the reader. The poem seems to see her softening as something he caused, which makes you wonder how much credit he deserves, and how she might tell the story herself.