I understand it driving people a little insane in Older and Far Away but the poor sweetie was feeling neglected in the worst way. After all within the space of two months she lost her Mom and her sister. Her entire family gone like that. Then she gets more than she could ever have hoped for- her sister back, alive and with her. And she barely looks at her for five minutes in one week.
Admittedly, they did all have lives, but Dawn needed a little love. She was a fifteen year old girl who'd gone through so much and just wanted for one night for everyone to be around her and not leave her alone at night eating leftovers (which she had been doing nearly every night for months).
The problem was Buffy and her friends were only 21 and while they loved Dawn they weren't able to give Dawn that love and attention she required. It was really the kind only a Mom could give and well, her Mom wasn't there anymore. So Dawn was acting out. Such loss and loneliness at such a young age was just too much for her. Hence the acting out.
And when they all find out they're stuck in the house I think it's Buffy's unfortunate phrasing that made Dawnr eact like she did. Buffy said 'We all have places we'd rather be' which implies that none of them want to be at the house where Dawn always is, ergo, they don't want to be with Dawn. If Buffy had said 'We all have places we have to be right now' it would have been quite different I think.