I guess Marianne's grief is consistent with her 'passionate' and emotional disposition. In the first chapter, the description of her and her mother's reaction to the passing of the father shows they both relish giving in to their emotions...

" Elinor saw, with concern, the excess of her sister's sensibility; but by Mrs. Dashwood it was valued and cherished. They encouraged each other now in the violence of their affliction. The agony of grief which overpowered them at first, was voluntarily renewed, was sought for, was created again and again. They gave themselves up wholly to their sorrow, seeking increase of wretchedness in every reflection that could afford it, and resolved against ever admitting consolation in future. Elinor, too, was deeply afflicted; but still she could struggle, she could exert herself. "