What does Laurie do that seems to hasten the romance between Meg and John Brooke? Why does this anger Jo?

Laurie wants to know what Meg's secret is. He knows it is something about John but Jo won't tell. John pens a letter to Meg under the guise of John expressing his passion for her. Meg of course is an excited school girl about this. Jo examines the letters and figures out they are forgeries. Jo of course doesn't like this deceit after she had refused to divulge anything to him.