We all know that Devil Fruit users weakness is the sea. But I wonder what is the conflict between sea and devil fruit? Why is the sea became the devil fruit's weakness?
"I once heard that all the Devil Fruits are the Sea Devil's incarnations. If you eat one you'll gain a special ability, but you won't be able to swim." ~Shanks to Buggy in Chapter 19, page 14
It seems that the Devil Fruits may be a physical manifestation of the devils of the sea as they try and give their powers to others, so the sea, in turn, hates all that partake in these powers and tries to weaken Devil Fruit users whenever they foolishly fall into its grasp.
It is said that the devil fruit if from the sea devil with no further complications the ocean refuses them and they lose their energy and power to swim.