Ron gave a yank on Dexter's leash to try to get him to stop slobbering all over Percy, but he was a very large dog and there wasn't much Ron could do when Dexter had made up his mind about something.
"Yeah, bet you wish you had thought of it first," Ron retorted angrily. His anger was probably irrational, but he didn't care. All he knew was that he couldn't stand there looking at Percy for another minute so he gave one last yank on Dexter's leash. It actually seemed to work that time and Dexter lost interest in Percy and gained interest in another person and dragged Ron off toward them.