I always thought you were immune to that question.  Wasn't it enough, looking into your eyes as I did?  But here is your answer, if you must have it:

What is beauty?  Is it pure sexual attraction?  Because I swear there is no alluring stranger, no old flame, no celebrity crush that I desire as I desire you.  Is it some arbitrarily magical combination of features?  Because since I have known you, you have been the measure by which all works of art are measured. Is it

Is it in the eye of the beholder?  Because your beauty in my eyes is not a matter of question or measure; it is the heartbreaking realization that I can never give up, knowing that the distance between myself and happiness can be measured in the miles between me and you.