Well, there's really nothing wrong with keeping a relationship to yourselves if that's what you really want, but do it for the right reasons.
I don't think your age would do it, and I don't know how to define it without sounding like an
. But you're all grown up, you know what it means. :P And in case you don't, here's a link to definitions (if you click it, don't blame me for what you read).
Aaaanyway, it sounds like you're just a private person. If you're just keeping this private because you kept your other relationships private too, then there's really no harm in telling others how much you love your girl. It's just a privacy thing that a part of you anyway. You have nothing to lose but more of her affection to gain.