Why is Rose so concerned about Lissa getting drunk?

Before the girls ran away, they were at a party, drinking. Lissa was making out with a senior boy, when another boy showed up with a feeder. The other boy went too far with the feeder, so Lissa threw him out. Worried what might happen anyways, Lissa followed the boy to his room, only to discover that he had taken advantage of her while she was under the effects of his saliva. Lissa lost control of herself, and used compulsion on the boy, making him destroy most of his own possessions with a baseball bat. Lissa was just about to make him bash out his own brains when Rose was able to talk her down.