"It was the period when historical novels with classical settings, ranging from the works of Mademoiselle de Scudéri to those of Madame Barthélemy-Hadot, high-minded in tone but increasingly vulgar in content, were indulging the romantic tastes of Paris concierges and penetrating further afield. Madame Thénardier had just sufficient intelligence to read books of this kind, and she devoured them, soaking in them what little mind she possessed."
As Éponine endured her first client the evening after visiting Grantaire, she marveled at the wide expanse of emotions and experiences a single day could hold. Not an hour before, she had been perfectly content – happy, even... But she still had to come home to this.
The next day, Éponine did not go to Grantaire. She went to Rue Plumet, where she watched Marius and Cosette from a concealed distance.
Why, one may well wonder? Why, after the evening's ordeal and reflections, would she not return to the one whom she'd noted had made her happy? Why go instead to gaze after a fantasy happiness which was inaccessible to her?
The answer is that a soul in pain needs a dream. It needs to set its sights on something or someone that it imagines would sweep all that pain away. For Éponine, a girl who'd learned to read on her mother's cheap romance novels, this dream was romantic love. And her dream of romantic love had of late taken the embodiment of Monsieur Marius.
The fact that Monsieur Marius was unattainable to Éponine only made the dream all the more powerful — all the more capable of bringing light to her darkness. An inaccessible dream has the advantage over an accessible reality, because we needn't live it. We needn't see how the dream materializes once it actually fits into the fabric of our life. In Grantaire, Éponine had accessed a real, true goodness. But surrounding that goodness was the rest of her life. Her friendship with Grantaire, however wonderful, did not efface the surrounding pain.
Should Marius have become attainable to Éponine, he too would have become merely a spot of goodness in her otherwise wretched world. Should he have become attainable, Éponine would have realized that Marius couldn't sweep away her pain anymore than Grantaire could.
But so long as he was out of reach, she needn't realize this. She could continue to sustain herself on her romantic dream.