I suppose one way to do this would be to examine the way in which love overrules judgement. This is an archetypal theme and is present in so many stories, mythological or otherwise. If we look at the story carefully, we see that the sudden love that is kindled between Mabel and Fergusson is something that completely overpowers them both, but in particular the qualms that Fergusson has about Mabel's class and social status. Consider the way that the doubts he has about their relationship are referred to in the short story:
The strange pain of his heart that was broken seemed to consume him. That he should love her? That this was love! That he should be ripped open in this way! Him, a doctor! How they would all jeer if they knew! It was agony to him to think they might know.
Fergusson recognises that falling in love with Mabel represents a poor match for him in the eyes of society, and he is full of shame as a result of her lowly status and how this will reflect upon him as a character. However, at the same time, the emotion of love is something that will not be defied so easily, and he recognises that, in many ways, he is love's slave, even though his reason disagrees with what he is going to do in marrying Mabel and coupling his life and prospects together with hers. This is a theme that is archetypal in the way that it occurs in so many works of literature.