The grammar point is, just like Breckenfeld has noticed THE goes with BRAIN here.   

     The human/body organs are reffered in this way, because they are regarded as unique: THE heart, THE brain et cetera.

      http://leo.stcloudstate.edu/grammar/useartic.html

Note: The is also used when what its noun refers to is unique.
     The White House 
     The theory of relativity
     The 1996 federal budget

   Now, my humble suggestions:

   The brain is an amazing organ, it works non-stop until the very end of your existence, or...until you fall in love!

   The brain is an amazing organ, it works non-stop until the very end of your existence, except...when the day comes that you fall in love!

    The brain is an amazing organ, it toils non-stop all the time, but where is the brain when you need him? I mean, right at the time when you fall in love?!

    Ha ha, nice topic!