If Charles only "imagines" that he loves Emily for her common sense, then why does he really love her? The narrator plays very coy with us here, suggesting a hidden reason that Charles shares in common with "the whole surveying camp," but not telling us what that reason is. To understand the hint, and the answer, one must understand Conrad's allegory, in which Charles represents materialism, and Emily altruism, in the abstract. In this allegory, materialism as such is in love with altruism; in other words it requires altruism as a humanizing consort-idea that enables it to believe that is essentially about uplifting downtrodden mankind. On its own, without the altruism consort, material progress would (and will) stand revealed as just another transformation of the Earth for the sake of power. See Charles' key line to Emily, "The best of my feelings are in your keeping." It is altruism that animates materialism as a moral crusade, that turns it into the notion of 'progress.' Materialism requires an altruist bride.
This is "exactly the reason" why the young railroad engineers, whom we have seen imbued with the idealistic fire of progress , revere Mrs Gould. Note that they remember her "convincingly," i.e., altruism convinces them in their materialist faith. Note elsewhere that everyone associated with the materialist cause, including Sir John, the Holroyd trio and Scarfe, reacts to her in the same way (, and ).
The story of the Goulds' marriage, which begins as an idealistic partnership
and ends in estrangement, is an allegory for the historical process by which
materialism gradually becomes separated from its moral justification, so that
it is no longer regarded as 'progress.' At the end of the book, when they are
estranged, Charles has become a cold automaton mechanically repeating the motions
of empty activity, and the populace has turned against the silver mine. The
allegory is hinted at in this passage, where, psychologically and allegorically,
the young engineers stand in for Charles' unadmitted self, and conclude the
sentence beginning with his name.