We always say "I love you" when we say good-bye. If anything were to happen to us, I want those to be the last words we say to each other. Even though we do it out of habit, we still mean it. We wouldn't say it if we didn't.
That's why i say it to my partners all the time. Spouse says it back and usually says it first, and Other never says it casually, but i still say it to Other and they don't seem to mind.