People go on marrying because they can't resist natural forces, although many of them may know perfectly well that they are possibly buying a month's pleasure with a life's discomfort.

The remark treats marriage less as a romantic ideal than as a capitulation to instinct and social momentum. “Natural forces” suggests sexual desire, the urge for companionship, and the biological drive toward pairing, while the transactional phrasing—“buying a month’s pleasure with a life’s discomfort”—casts the institution as a bargain many enter despite foreseeing its costs. In Hardy’s characteristic pessimistic realism, brief intensity (courtship, early passion) is contrasted with the long duration of domestic compromise, incompatibility, and obligation. The line also implies a tragic lucidity: people may understand the likely outcome yet proceed anyway, highlighting the limits of rational choice when desire and convention exert pressure.