This is potentially the most baffling book adaptation that somehow makes it necessary to experience both.

Spoilers obviously

So in the book Sean and Lauren do hook up and start dating. Things in there relationship just end up getting worse and worse for them until it's over.

What the movie does is completely flip this. Sean wants that relationship he had in the book but can never achieve it. Lauren ends up despising him and never giving him a chance. And regardless of either situation Lauren and Sean leave more miserable then they could have been.

Paul and Sean hook up in the book and the exact same shit happens. They end up more miserable then before.

The movie and book together give the message that no matter which decision or action these characters take they'll continue to be miserable. It makes the book infinitely more depressing. I don't think an adaptation has ever done that.