This might turn out to be vacuous, but it seems useful to me. Here goes nothing:

Do you have a favorite color? Or a favorite number, or word, or shirt, or other arbitrary thing? (Not something that's a favorite because it reminds you of something else, or something that you like because it's useful; something that you like just because you like it.)

Assuming you do, what objective value does it have over other similar things? None, right? Saying that purple is a better color than orange, or three is a better number than five (to use my own favorites) simply doesn't make sense.

But, assuming you answered 'yes' to the first question, you still like the thing, otherwise it wouldn't be a favorite. It makes sense to describe such things as fun or beautiful, and to use the word 'happiness' to describe the emotion they evoke. And you can have favorites among any type of things, including moral systems. Rationality doesn't mean giving those up - they're not irrational, they're arational. (It does mean being careful to make sure they don't conflict with each other or with reality, though - thinking that purple is somehow 'really' better than orange would be irrational.)