Valentine's Day - much like Halloween - is a "holiday" that Germans haven't celebrated until recent years because of their imitation of the American tradition. It was not a holiday I celebrated as a child with my German family in any case.
I won't attempt to write much about Valentine's Day in Germany as many other German-blog-writers have already tackled the subject:
I am just popping by to say ALLES LIEBE ZUM VALENTINSTAG!
And to mention a few ways to express LOVE in German ---
There is the simple Ich liebe dich. = I love you.
There is the more German-native-sounding - Ich hab' dich lieb. Literally it is translated "I have you love" but perhaps it makes more sense as thinking of it as "I have love for you." In my experience and from what I understand, this phrase expresses fondness - it is said to family members, maybe a close friend even. It more means I am very fond of you.
In contrast, Ich liebe dich, for a German is a little more serious and might be best reserved for a boy/girlfriend you are serious about or a spouse or significant other. A child might say it to her parents as well.
If you are German, what is your experience?
Again, in my own experience, my German family didn't often express our love by a phrase equivolent to I love you. Instead, we might close a letter Mit Liebe Grüsse - with Loving Greetings. Or Alles Liebe with all my love.
In reading up on these phrases in a more modern, less verklempt Germany, teenagers now may text HDL to their closest friends. That's short for Hab' dich lieb! It's kind of like saying "Love ya" in English. Also the abbreviation hab' is lazy German; it is a shortened form of the verb habe (to have).
Finally, for your Valentine, I will tell you some of my favorite German romantic nicknames. If you're tired of "honey" and "darling", try these:
1. Schatz (also, Schatzi) - let them know they are your TREASURE
2. Sweet animal nicknames: Maus (Mausi, Mäuschen), Hase, Bärchen (mouse, rabbit, little bear) or even Mausebär (mousebear), which is, of course, a made-up name/animal but sounds cute.
3. There's also Schnecke (snail) ... or another kind of made-up word form of this: Schnucki
4. Most people will recognize the word Liebling, which is the German equivalent of darling.
5. Süsse (for females) or Süsser (for males), which means Sweetie.
Happy Valentine's Day from