Ba’erev tochlu matzos; “In the evening you shall eat matzos”. The word erev (evening) is used which also means sweet. This is to teach us that the eating of the matzos should be with pleasantness and sweetness. It is sweet to the pallet and pleasant to the soul because it is a mitzvah that comes but once a year.
(תפארת שלמה מהרבינו שלמה מראדמסק עמ’ ח )