Kedushas Levi Parshas VaYera - Why did Avraham feed the angels meat and not fish?
The Kedushas Levi answers that he wanted to train them inmitzvos (Bereshis Rabbah 48:13) and meat (and it's preparation) has many mitzvos associated with it such as shechitah – kosher slaughtering, melichah – salting, Nikur – removing forbidden fats. However fish have no mitzvah associated with them (in their preparation).
However in the future (during the time of the Moshiach) the primary feature course of the festive meal will be fish. And this is why the future festive meal is known as the meal of Leviathan - Seudas Livyasan. (Bava Basra 75a)
Stories
Why
Not Feed Them Fish
During the great wedding that took place in Zlovin when the daughter of the Mittler Rebbe, the granddaughter of Rav Shneur Zalman of Liadi the Alter Rebbe of Chabad married Rav Levi Yitzchak of Berditchev's grandson, the chuppah took place as was the custom in those on Erev Shabbos.
On Friday morning the Alter Chabbad Rebbe's wife entered his room with complaints. “All of the preparations for the meals are all on my head, (as was the custom that the bride's family handles the meals) and what can I do there is no fish to be bought at the market! And what's worse,” she continued, “the chassidim told me that if the mechutan Rav Levi does no have fish, he won't eat anything else at the meal at all!,” she concluded. “If that is the case,” answered Rav Shneur Zalman, “please go and complain to the mechutan directly, state your case and explain the situation, because I do not get involved in such matters.”
The Rebbitzen came before Rav Levi Yitzchak and explained that she could not prepare any fish since there was none to be bought at the market. “Is there no river in these parts?” asked the Berditchiver out loud. When the chassidim answered that the Dnieper was close by he ordered a coach and wagon and asked them to call his mechutan the Alter Chabbad Rebbe to join him on the fishing expedition.
When they reached the Dnieper river Rav Levi Yitzchak removed his handkerchief from his pocket and swished it about, to and fro in the water. After doing so several times, many fish began to swim towards them from all directions. There was nothing left to do but to fill the buckets and barrels they had brought with the fish and go home!
(Migdal Oz – Masay Avosai #23 p.165)