(ב) לוקח מן הכרפס פחות מכזית - כדי שלא יתחייב בברכה אחרונה - טובל במי מלח, מברך "בורא פרי האדמה", ומכווין לפטור בברכה גם את המרור. אוכל בלא הסבה.
This is the directive for Karpas eating. Note, however, that we make sure that Karpas has a Bracha Rishonah but not a Bracha Acharonah. Why not just eat an olive-sized piece and say a Bracha Acharonah?
And seeing as blessing on food are considered blessings of enjoyment, we can add to our query: What is the point of dipping Karpas into salt-water, which also minimizes enjoyment?
To answer this question, we need to find out the true meaning of Karpas.
(ו) ח֣וּר ׀ כַּרְפַּ֣ס וּתְכֵ֗לֶת אָחוּז֙ בְּחַבְלֵי־ב֣וּץ וְאַרְגָּמָ֔ן עַל־גְּלִ֥ילֵי כֶ֖סֶף וְעַמּ֣וּדֵי שֵׁ֑שׁ מִטּ֣וֹת ׀ זָהָ֣ב וָכֶ֗סֶף עַ֛ל רִֽצְפַ֥ת בַּהַט־וָשֵׁ֖שׁ וְדַ֥ר וְסֹחָֽרֶת׃
(6) there were hangings of white, fine linen, and blue, bordered with cords of fine linen and purple, upon silver rods and pillars of marble; the couches were of gold and silver, upon a pavement of green, and white, and shell, and onyx marble.
In calling the greens 'Karpas' instead of 'yerek', we are connecting it to the feast of Achashverosh. But of what significance is the feast?
"In those days, when the king sat [on his throne]" (Esther 1:2). But it says just afterwards, “In the third year of his reign?" Rav said: What is meant by “when he sat”? After he began to feel secure. He reasoned thus: Belshazar calculated and made a mistake; I have calculated and made no mistake.
The feast of Achashverosh took place as a sort of victory party over the Jews (though he also was wrong with his count). Thus, Achashverosh is an enemy of ours- and the drama of the Megillah can be seen as the battle to defeat his anti-semitism, which is ultimately successful. He switches sides, and Haman as well as the other antagonists are also killed.
Purim is all about celebrating the defeat of anti-Semitism. However:
(יז) בִּנְפֹ֣ל אויביך [אֽ֭וֹיִבְךָ] אַל־תִּשְׂמָ֑ח וּ֝בִכָּשְׁל֗וֹ אַל־יָגֵ֥ל לִבֶּֽךָ׃ (יח) פֶּן־יִרְאֶ֣ה יְ֭הוָה וְרַ֣ע בְּעֵינָ֑יו וְהֵשִׁ֖יב מֵעָלָ֣יו אַפּֽוֹ׃
(יט) שמואל הקטן אומר, (משלי כד) בנפל אויביך אל תשמח ובכשלו אל יגל לבך, פן יראה יהוה ורע בעיניו והשיב מעליו אפו .
(19) Shmuel the Younger quotes [as his motto]: (Proverbs 24:17-18) "When your enemy falls, do not be happy, and when he stumbles, let your heart not rejoice. Lest God see and it be bad in His eyes and He turn from him [the enemy] His anger."
Purim is special in that it is based on the defeat of our enemies, and thus we can celebrate that. But Pesach, although it also contains the spectacular defeat of our assailants, is not all about celebrating their demise. Thus, the above adage applies, and so when we remember those who failed to destroy us, we must tone down the enjoyment.