Josh31, reason “a” is for adult babies(the parents)only.
Reason “b” is a beautiful reason to show our excitement over this mitzva.
Here’s my reasoning for reason “c”: Boys entire LIFESTYLES change at age 13. Their chiuv to perform mitzvos makes their lives more demanding. From dressing like adults in suit and hat, to putting on teffilin, making sure to daven with a minyan, expecting them to act with more maturity… There’s a lot more that’s expected from a bar mitzvah boy (and older) than from a bas mitzvah girl regarding the performance of mitzvos.
A girl doesn’t cover her hair at that age, isn’t required to perform mitzvos shehazmon grama, her lifestyle doesn’t change. And because of the demands that are placed on a bar mitzva boy from one day to the next, we make it exciting and special. We make him have enthusiasm towards his coming of age so that he will take upon all the new mitzvos that he will perform with excitement.
That’s why we make we celebrate when we make a wedding, btw. Setting up a family is much more demanding than being single. From a husband (or wife – but is should really be the husband’s department) providing for his family, having children, bringing up children… It’s certainly much more burdensome than being single. So the wedding celebration is all about injecting excitement and to show that’s it’s an amazing time where we are building another binyan aday ad.
Lgabeh lavish and ostentatious simchos is another story. First of all, Jews have to act betznius in all aspects of our lives, even if they have all the money in the world. Tznius with our houses, simchos, dress and action.
Seperately from this is the crazy, needless pressure to keep up withe the “Cohens”. As my shitta is the middle way, I wouldn’t recommend making simchos in the house if others in the community do make it in a hall. I do believe that going the total opposite of ostentatious and making it in a very poor manner can also have a negative effect. So a simple hall, nicely, but not extra lavishly decorated, and excitement from the heart is a perfect recipe for perfect simchas. Simchas are excatly what the word implies – it’s a time for simcha.
As for bas mitzvas, I personally think a nice, women only type small celbration in one’s home, or a celebration for immediate family, also in the home, not in a hall, is a beautiful idea. I don’t see anything wrong with that. I think it’s a beautiful thing to show excitement at the time when a girl becomes mchiuv to do the mitzvos.