For a girl we make a private party, as a tribute and haschala for her being mechuyav in mitzvohs, specifically in areas of tznius. What a befitting way – a party in a tznua way.
For a boy we throw a lavish celebration to mark his being mechuyav in mitzvohs, to show how happy we are with HaShem’s mitzvohs, we’re readily accepting them upon ourselves, now as a full-fledged “metzuvah v’oseh”. HaShem’s Mitzvohs – that’s something worth getting excited about.