Is not the motive behind the stiff upper-lip the idea that it is wrong to impose one's own feelings on others?

As for Darcy expressing oneself, this may vary depending on your view of how much a man violently in love can be supposed to. Clearly I have higher expectations of a man violently in love than you do. And, regardless of our individual views on the matter, clearly Darcy speaks far more than Elizabeth can at this juncture.