It is difficult at times to watch Marianne indulge in excess sensibilities and hear some of her opinions yet still consider her something of a rational creature but as I am extremely fond of Marianne try I must. (;D)

I think the excess sensibilities in which Marianne and Mrs. Dashwood indulge are scripted in the sense that they have imbued what true heartfelt grief (or happiness) looks like from romantic stories and poems. Apparently to have strong emotions and suppress the expression of them is wrong and further the expression of feelings must do justice to the strength of them:

Marianne abhorred all concealment where no real disgrace could attend unreserve; and to aim at the restraint of sentiments which were not in themselves illaudable, appeared to her not merely an unnecessary effort, but a disgraceful subjection of reason to common-place and mistaken notions. (Ch. 11)

In Ch. 4 Marianne scolds Elinor for stating her feelings “in so quiet a way” saying “Cold-hearted Elinor! Oh! worse than cold-hearted! Ashamed of being otherwise” suggesting her lack of raptures about Edward are due to a wish to hide her feelings. When Elinor did not act as Marianne expected a woman in love ought when parting from her lover she is puzzled:

"And Elinor, in quitting Norland and Edward, cried not as I did. Even now her self-command is invariable. When is she dejected or melancholy? When does she try to avoid society, or appear restless and dissatisfied in it?" (Ch. 8)