Well in order to make good wine some grapes may need to be sour.
Another thing is lemonade- both bitter and sweet and delicious, mixed together and blending perfectly, sort of like when you love someone.
I would end up saying that one of the most delicious bitter sweet things in life are Life Savers candies. If you compare them to love, they are sweet, sour, they melt, and they make you feel young again.
It could help ;)