Here is a love letter that I like, written by Rilke:
To love is good, too: love being difficult.
For one human being to love another: that is perhaps the most difficult of all our tasks, the ultimate, the last test and proof, the work for which all other work is but preparation.
For this reason young people, who are beginners in everything, cannot yet know love: they have to learn it.
With their whole being, with all their forces, gathered close about their lonely, timid, upward-beating heart, they must learn to love.
But learning-time is always a long, secluded time, and so loving, for a long while ahead and far on into life, is--solitude, intensified and deepened loneliness for him who loves.
Love is at first not anything that means merging, giving over, and uniting with another (for what would a union be of something unclarified and unfinished, still subordinate--?), it is a high inducement to the individual to ripen, to become something in himself for another's sake, it is a great exacting claim upon him, something that chooses him out and calls him to vast things.