In Letter 2, Robert continues to explain his passion for knowledge, but he is depressed not only because of the frost and snow, but more so because he?

He is depressed because he has no one to share his success with.

"I have no friend, Margaret: when I am glowing with the enthusiasm of success, there will be none to participate my joy; if I am assailed by disappointment, no one will endeavour to sustain me in dejection."