Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Volatile, Romantic, Critical, Candid, Cathartic
Technically, Daniel Handler wrote this novel, but in the world of the story, the author of what we're reading is Min. (Remember: The text is a letter she's writing to Ed.) Breakups bring up a lot of feelings, so Min's on a bit of an emotional rollercoaster—and before she gets to the scene of the breakup itself, much of what Min writes about is the good part of the relationship. She's recalling the happy times from a place of deep anger, though, so tonally, the book is all over the place. She's happy, she's sad, she's in love, she's furious; she misses Ed, though she knows that she shouldn't.
As she writes, Min tracks how she felt (past tense), how she feels now, and how there's still some overlap between the two. In doing so, she paints a pretty complete picture of her experience of being with Ed.
The breathless romantic quality of describing her first love is balanced by Min's critical faculties, which she directs mostly toward Ed, acknowledging his good qualities (hotness, mostly) and terrible faults. She does the same for herself, noting her own failures as a friend (to Al and Lauren) and to herself (for ignoring the signs that something was wrong). It's a pretty earnest approach she brings to the whole assessment process.
Still, toward the end of the letter, there is a leveling off—her tone becomes less angry, romantic, and wistful as she starts to look toward the future. Consider the next-to-last chapter, which is only two sentences: "Give this back to your sister. I'm done" (42.1). After hundreds of pages of emotional purging, Min seems ready to let go.