Why did Mathilde weep with misery after visiting her schoolmate?

Mathilde feels herself entitled to better things in life. She leads a pretty ordinary, lower middle-class existence as the wife of a minor official at the Ministry of Education. Although she herself comes from a family of clerks, Mathilde genuinely believes that she has noble blood in her veins and often fantasizes about living a charmed life among the aristocracy.
But the huge gulf between fantasy and reality is starkly illustrated for Mathilde whenever she goes to visit her rich friend. Witnessing at firsthand the luxurious lifestyle that her old school friend leads brings home to Mathilde just how far away she is from living out her dreams. This makes her incredibly upset, so much so that she weeps with sorrow for days upon returning home. So deeply traumatized is Mathilde by these visits that she stops going to see her friend altogether.