It is stated in the Talmud (Kiddushin 81a): "Bitter herbs do not require a reason." Rashi and Rashbam explained that this is because it is a reminder of slavery. On the surface, it seems obvious that bitter herbs do not require a reason, so what is it teaching us? Perhaps, if one seasoned their eating with it, they do not fulfill their obligation, because seasoning is a manner of freedom, and it contradicts the directive of bitterness, which is a reminder of slavery. Indeed, Beit Yosef (Siman 496) wrote in this language, "Bitter herbs do not require a reason," and Rashi explained, "because it is a reminder of slavery." It seems to me that if one wanted to eat it seasoned, they are allowed to do so.

The Beit HaLevi further explained (letter Zayin), "Since when the Temple stood, the reason was necessary because of the Passover sacrifice, which is a reminder of freedom, as it is written (Numbers 9:11), 'They shall eat it with unleavened bread and bitter herbs,' implying that eating the Passover sacrifice is the main thing, thus currently we also need to do something to commemorate the eating of the Passover. Additionally, the Beit Yosef wrote to strengthen this opinion, that we only say that bitter herbs do not require a reason, but if one wants to season it, they may do so and there is no prohibition." This means that bitter herbs also serve as a reminder of the Passover sacrifice, and during the time of the Temple, they ate the Passover with matzah and bitter herbs seasoned. Thus, according to Jewish law, there is no need to season the eating of bitter herbs, but if one does, it does not hinder their obligation. The Rambam also did not mention at all that if one eats bitter herbs seasoned, they do not fulfill their obligation, implying that it does not hinder.