In the first place, Catholics do not HAVE to eat FISH on Fridays. They may eat anything they like EXCEPT MEAT.

Friday is a day of abstinence from meat for Catholics in order that this little sacrifice will be a work of satisfaction for the sins they have committed. The Church COMMANDS IT just as a, mother will insist on her child making restitution for something the child may have stolen; or will insist on eating nourishing food. The Church is a mother and knows that unless we are constantly reminded we will not make satisfaction for our sins.

Meat is not considered as something BAD by Catholics. If so, the Church would certainly not permit it on every day EXCEPT one. It would be bad EVERY day.

Friday is chosen because this is the day on which our Lord died to atone for our sins and to remind us that we should offer our sacrifices in union with His sacrifice on the cross.