Many Deaf learned to use the sign language since they are kids. Many Deaf adults also tell the kids with use a sign "I love you". That's why the Deaf kids remembered this sign until they are growing up!
I don't remember that where I get to learn from since I am kid... Haha
How to make "I love you" in a sign? See the more information about StartASL.com
If you are hearing person, you shy to tell "I love mom" to your mom? You can try to use this sign! It is fun to show your sign when you are learning the sign language!