What guy who's getting ready to get laid runs out and buys a new diary for his girl with no notice? Couldn't he have waited? I know it was done for dramatic effect so we think "oh no", but please. No man, even as civilized as Mr. Darcy would leave in that manner to buy a new diary BEFORE sex.
"George is dead. Call me back." RING. Hello? "Hi. It's George."
What was really stupid was that he actually read her diary at all. Totally rude. I also thought that it was stupid that he just bolted while she was changing, without even letting her know. Like you said, it was played for dramatic effect, and I guess we were supposed to be happy that he was coming back, but in real life any girl would be like "what the hell, dude?".
Because she thought she had lost the man she loved, and she was desperate to explain things to him and make things right. She didn't know that he was merely walking to a store to purchase a new diary for her. When you're desperate, you don't exactly think straight.
"Why would she run after him in her underwear, that was tacky on her part. At least put some clothes on."
LOL, am I the only other person who is amused by the fact that its Mr. Darcy stepping out to purchase a diary - and NOT her leaving the house with NO PANTS in a snowstorm - that is stretches credibility?