At times one does something so often that he stops thinking why he is doing it. I am glad to pause for a moment to understand this staple of our Friday night meal.
The prophet Isaiah (58:13) tells us "and you will proclaim the Sabbath 'delight,' and the holy [day] of G‑d 'honored.'" Our Rabbis explain that one honors the Shabbat by wearing special clean clothes and has pleasure on Shabbat by enjoying delightful food and drink on this day.
In the days of the Talmud, the food that was considered delightful and was used for this purpose on Shabbat was fish. Meat and wine were added to the Shabbat meal since most people enjoy these foods too. One can and should eat any specific Kosher food that they consider pleasurable.
In addition to this there are interesting reasons given for this custom. G‑d blessed three things consecutively as they were created. He blessed fish on Thursday, man on Friday, and the Shabbat on Saturday. "A three-stranded cord will not quickly be broken." Thus, we combine the three: 'man' eats 'fish' on 'Shabbat' thus causing that he should be blessed with the threefold priestly blessing:
May the L-rd bless you and watch over you.
May the L-rd shine His countenance upon you and grant you grace.
May the L-rd raise His countenance toward you and grant you peace.
Additionally, fish are creatures that do not have eyelids and thus never close their eyes. This is a metaphor for G‑d, whose eyes and gaze are always upon those who follow in His way to administer to them with His great mercy.
All the best,
Rabbi Shmuel Kogan,