Halocho #1686 - Why do the firstborns fast - shouldn't they celebrate?

Why don't firstborns celebrate the fact that they were saved from the 10th plague - the smiting of Egyptian firstborns?

On Erev Pessach - Friday next week - all firstborns will fast in memory of them fasting in Egypt on Erev Pessach, to ensure they wouldn't be punished along with the Egyptians in the 10th plague.

The Hallel said at the Seder includes praise for their delivery.

All firstborn males fast; even if they're only a firstborn to one of their parents.

Even firstborns who are exempt from Pidyon haBen - like Cohanim, Levites or being born after a stillborn - also have to fast.

The father of a young firstborn needs to fast for him.

Whether a firstborn (or his father) may attend a סְעוּדַּת מִצְוָה like a Siyum, Bris or Pidyon haBen and break his fast to participate in the meal, depends on local / family custom.

After breaking his fast, he can eat the rest of the day.

Source: Kitzur Shulchan Aruch 113:6

- Danny
Tuesday, 4 Nissan 5776

הַבְּכוֹרִים בֵּין בְּכוֹר לְאָב בֵּין בְּכוֹר לְאֵם, מִתְעַנִּים בְּעֶרֶב פֶּסַח, אֲפִלּוּ חָל בְּעֶרֶב שַׁבָּת. וְגַם הַבָּא אַחַר הַנְּפָלִים, צָרִיךְ לְהִתְעַנּוֹת. וְכַל זְמַן שֶׁהַבְּכוֹר קָטָן, הָאָב מִתְעַנֶּה תַּחְתָּיו. בִּסְעוּדַּת מִצְוָה אִם מֻתָּרִים לֶאֱכוֹל, תַּלְיָא בְּמִנְהַג הַמְּקוֹמוֹת