Here there are some messages that you can write on St Valentine's Day cards:
1) Sending you love and hugs on Valentine's Day to let you know that you are very special to me. Happy Valentine's Day!
2) Thank you for always being a true friend. Happy Valentine's Day!
3) A friend like you is a precious gem... hard to find. Happy Valentine's Day!
4) To a friend who is always close to my heart.
5) Just wanted to say Hi with a warm hug and wish you a very Happy Valentine's Day!