Although normally one best fulfills a negative commandments by saying: "It is certainly tempting to do [or enjoy] this bad thing, but what can I do, Hashem forbade me to do [or enjoy] it" [see Rambam, Shemoneh Perakim], this is not the case in regard to chametz on Pesach. Thus, during Pesach a person should not say: "I am certainly tempted to eat and drink chametz, but cannot do so because of Hashem's command." This is because chametz on Pesach is akin to avodah zarah [Zohar 2:182a; see also Pesachim 28a and Shelah, Pesachim, Perek Torah Ohr #13], and concerning avodah zarah the Torah states: V'lo yishama al picha - "It [the name of an idolatry] should not be heard upon your lips." Hence just as it is forbidden to mention, and even think of, an idolatry, so too on Pesach one should avoid mentioning, and even thinking about chametz. Rather, before Pesacha person should say: "How much would I like to eat chametz on Pesach, for it is good and tasty, but what can I do, Hashem forbade me etc."
(Heichal Berachah (Kumarana), Parashas Re'eh on the pasuk: "Ki yirchak mimcha hamakom," cited in Minhag Yisrael Torah, Orach Chaim 469:2)