I think people say "I love Jehovah" because it helps them express their beliefs. It's in the name "Jehovah's" Witness, so I think it's also a group bonding, like with early Christians and the fish. They are a group that are separate from all other groups and their is a standing prejudices about Jehovah's so by saying that or writing it, it opens a door to like-minded people who they could possible get to know.
I could be really wrong, this is just what I think. The only time I have seen it was written on a friend's car from a fellow church member, but this is my guess.