What are some bitter and sweet things?something that needs the bitter aspect to make it sweet and desirable... something that could be compared...

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 ;)