Have you ever gone on vacation with someone when you weren't feeling 100 percent excited about it? It's not a plan that most people consciously enter into, because it's just a plain bad idea. If you're going to be with someone in a foreign location for 24 hours a day (and most probably sharing a tiny hotel room with them, too), you want them to be pretty special.
That's how you know she's in love with you. The moment she starts talking about going on a romantic vacation together, you know she's head over heels. You don't try to go on a couple's trip if you're only half-hearted about the relationship. This one is a double whammy, because it's a dead giveaway that she really loves you AND you get to start planning that sweet trip to the Bahamas you've been dreaming about.