They seemed to come suddenly upon happiness as if they had surprised a butterfly in the winter woods.

The simile frames happiness as something delicate, rare, and almost out of season—like a butterfly improbably encountered in winter. The characters do not “achieve” happiness through planning or entitlement; they stumble upon it, and the verb “surprised” suggests both wonder and the risk of harming what is found. Wharton often treats joy as contingent and socially or psychologically fragile, and this image implies that happiness is brief, easily lost, and not fully compatible with the surrounding “winter” atmosphere (emotional coldness, constraint, or hardship). The line captures the poignancy of an unexpected reprieve rather than a stable state.