Why should all natures be alike?  It would make the old saws useless if they were, and deprive us of one of the truest of them all, ‘Variety is the spice of life.’  How terribly monotonous it would be if all the flowers were roses, every woman a queen, and each man a philosopher.  My private opinion is that it takes at least six men such as one meets every day to make one really valuable one.  I like so many men for one particular quality which they possess, and so few men for all.  Comprenez-vous?

In another place: 

Is it not a trifle dangerous, this experiment we are trying of a friendship in pen and ink and paper?  A letter.  What thing on earth more dangerous to confide in?  Written at blood heat, it may reach its destination when the recipient’s mental thermometer counts zero, and the burning words and thrilling sentences may turn to ice and be congealed as they are read. . . .  A letter; the most uncertain thing in a world of uncertainties, the best or the worst thing devised by mortals.

Again: 

Surely it was for you, mon cher, that the description given of a friend of mine was originally intended.  He is a trifle cynical, this friend, and decidedly pessimistic, and of him it was reported that he never believed in anything until he saw it, and then he was convinced that it was an optical illusion.  The accuracy of the description struck me.

They seem to have loved each other best when they were parted.

I think I cannot bear it much longer, this incessant quarrelling when we meet, and your unkindness during the short time that you are with me.  Why not let it all end? it would be better for both of us.  I do not love you less when I write these words; if you could know the sadness which they echo in my heart you would believe this.  No, I think I love you more, but I cannot understand you.  As you have often said, our natures must be very different, entirely different; if so, what is this curious bond between them?  To me you seem possessed with some strange restlessness and morbid melancholy which utterly spoils your life, and in return you never see me without overwhelming me with reproaches, if not for one thing, for another.  I tell you I cannot, will not, bear it longer.  If you love me, then in God’s name cease tormenting me as well as yourself with these wretched doubts and questionings and complaints.  I have been ill, seriously ill, and there is nothing to account for my illness save the misery of this apparently hopeless state of things existing between us.  You have made me weep bitter tears of alternate self-reproach and indignation, and finally of complete miserable bewilderment as to this unhappy condition of affairs.  Believe me, tears like these are not good to mingle with love, they are too bitter, too scorching, they blister love’s wings and fall too heavily on love’s heart.  I feel worn out with a dreary sort of hopelessness; if you know a cure for pain like this send it to me quickly.

Yet, in the very next letter, she says to him: