Love is all the justification you need for sending a love letter.
I'm not sure that I understand your question, but I gather that you're looking for quotes about love letters and the reasons for sending them. Here are a few:
"One night when there was a clear moon, I sat down to write a poem about maple-trees. But the dazzle of Moonlight in the ink blinded me, and I could only write what I remembered. Therefore, on the wrapping of my poem I have inscribed your name." — Amy Lowell (1874-1925), American poet and critic
"Fill your paper with the breathings of your heart..." — William Wordsworth, English poet, to his wife, Mary, April 1812
"I'd love to spend all my time writing to you; I'd love to share with you all that goes through my mind, all that weighs on my heart, all that gives air to my soul; phantoms of art, dreams that would be so beautiful if they could come true." — Luigi Pirandello, Italian dramatist, to Marta Abba, an actress, Jan. 1926
"When you have received a letter you first of all sit down; cutting open the envelope is done slowly and resolutely, as though diffidently raising the lid of an enchanted chest." — Karel Capek (1890-1938), Czech journalist and writer
"What man does not remember the composition of his first love letter? The careful writing, the chosen paper, the blameless pen, the repeated failures before final perfection, the tender solitude which allowed no intervening hand to bear the laboured missive to the post office, and the longing anxiety which awaited a favourable reply...
"What woman can forget the arrival of her first billet doux? The expectant heart, the trembling fingers, the delight of reading young love's artless revelations in some secret corner, where no profane eye might see her, and where her imapassioned lover might urge his suit, uninterrupted by any voice unsympathetic with that sweet time." — J.T. Merydew, 19th century English writer (anthologist of love letters)
"I will touch
My mouth unto the leaves, caressingly;
And so wilt thou.
Thus, from these lips of mine
My message will go kissingly to thine,
With more than Fancy's load of luxury,
And prove a true love letter."
— J.G. Saxe (1816-1887), American writer
These quotes, along with many more, can be found in "Passionate Love Letters: An Anthology of Desire" and "Love Letters: An Anthology of Passion", by Michelle Lovric. Great books.