Before we started going out I would say it in a playful manner. When we started going out I started saying it more heartfelt. Even now that we're separated I still say it to her before leaving to work & before going to sleep, meaning it with all my heart
It's different for everyone. Personally it's extremely difficult for me. I doubt I will ever feel for another girl the way I do about my cousin. You can love other people. But there's always that one that will always have your heart. For me it is her.