It is not necessary to share your pack of french fries or your portion of fresh shrimp with seagulls. They do have food enough. If they are constantly fed by humans, they will no longer go find food themselves. They will tear garbage bags open which causes the street to be full of litter and which attracts vermin. Rooftops, cars and sidewalks are covered in seagull excrement. Moreover, seagulls are known to come and steal food out of people's hands, which can prove dangerous, especially for small children.
Did you know that is forbidden by law to feed seagulls? If you go ahead and do it anyway, you risk a heavy fine in most coastal municipalities along the Belgian coast.