The Torah commands us to be joyful on Shavuot - "You shall rejoice before the L-rd your G-d," and on Sukkot - "You shall be joyful before the L-rd your G-d," "You shall be happy" - but there is no specific commandment for Passover. This is because an entire nation (Egypt) was punished; G-d said, "My handiwork is drowning in the Sea." Thus our joy on Passover is tinged with sadness and therefore incomplete.
(Tanya Rabati)