Being very close to her in age I can understand her passionate ways and irrational thinking but her problem is that she doesnt know she is being irrational. Eleanor has the right idea and controls herself and if her thoughts are ever overdramatic she knows it and keeps them under control, but Marianne just doesnt know that you can be over dramatic, she thinks thats how people are supposed to be so she just revels the passionate feelings that when left untamed without reason or wisdom run wild as is clearly demonstrated here.