The loveliest one I've read is from author John Steinbeck to his teenage son, who has fallen in love at boarding school.
An excerpt from the beginning:
First—if you are in love—that’s a good thing—that’s about the best thing that can happen to anyone. Don’t let anyone make it small or light to you.
Second—There are several kinds of love. One is a selfish, mean, grasping, egotistical thing which uses love for self-importance. This is the ugly and crippling kind. The other is an outpouring of everything good in you—of kindness and consideration and respect—not only the social respect of manners but the greater respect which is recognition of another person as unique and valuable. The first kind can make you sick and small and weak but the second can release in you strength, and courage and goodness and even wisdom you didn’t know you had.
My second favorite part:
You say this is not puppy love. If you feel so deeply—of course it isn’t puppy love.
But I don’t think you were asking me what you feel. You know better than anyone. What you wanted me to help you with is what to do about it—and that I can tell you.
Glory in it for one thing and be very glad and grateful for it. The object of love is the best and most beautiful. Try to live up to it.
And don’t worry about losing. If it is right, it happens—The main thing is not to hurry. Nothing good gets away.
Nothing good gets away. I just adore that line so much, and I think of it often.
You can read the full letter here.