The two of you have decided to run away together. Bottom line, you've come up with that plan and you've worked up the guts and the means to do it. The most obvious source of a desire to leave together is a romance you can't have until you're gone from this place. It's like all the eloping that happened in those epics of old. How charmingly beautiful! How romantic! Who says the old ways have gone out of style?
But other reasoning behind the decision could be from the serious - you both want to escape imprisonment, an oppressive atmosphere, or a miserable destiny - to the more lighthearted. Maybe an adventure is just what you need, where you'll finally get to see the world...and, slowly, fall in love. Because that's what the crux of all this is: even if the escape started out as mere convenience, you both want to stay close and work together. In the end, when you've finally run where you want to be and the danger, if any, has passed, you may find you don't want to go your separate ways.