Class-crossed lovers (as Professor Burstein says). Though this crossing here is not as loud or vivid as it was in later Romantic pieces, including even the Bengali novel 'Devdas.' 
 There is something more fundamental in Manon Lescaut. It has to do with love's fight with the world, or perhaps with the struggle that an ideal love faces with the real world. 
 Some of the challenges are: 
 1. The demeanor of the loved one - the subjectivity of the object of my love may thwart whatever ideals I've based my love on 
 2. Money as the fuel of sustained togetherness - this is where economics enters the idea 
 3. The palpable cynicism of the real world regarding anything ideal 
 4. The palpable antagonism of the values of the world against the value of love 
 5. Other lovers for the loved one 
 All are dealt with here. Sometimes these causes commingle to create the singular effect of lovers' separation, but what is constantly suggested is the impossibility of love's real manifestation. 
 One has to see the usual love cliches, those that advocate a selfless pure ideal love, in a new light after this book. Do these aphorisms exist precisely because love's worldly manifestation is too difficult? Are they not pure ideas in themselves but mere consolations? 
 Anyhow. 
 The best line I've ever read on the topic still remains a line from a 21st century novel 
 "Love me because it doesn't exist, and we've tried everything that does." -- Jonathan Safran Foer in 'Everything is Illuminated'