Why do bar mitzva boys publicly read from the Torah for their bar mitzva? Where does the minhag come from?
Taken from Matazav.com The source is found in the Orchos Chaim - a French Rishon. He mention s that Rav Yehudai Gaon made a Baruch Shepotrani after his son leined by his Bar Mitzvah. He also mentions that this is done to show that the Bar mitzvah boy is a Gadol & can be a Shliach Zibbur
This tradition is actually becoming less common in the more "yeshivish" communities. The idea is that the enormous amount of time and effort spent memorizing the leining could be better spent learning gemara. This is probably true, but on the other hand, if you learn how to lein instead of just memorizing, that is also a very valuable skill and benefits the community greatly. Perhaps a compromise is to have the B"M boy lein just an aliya or two and also make a siyum on a masechta.
But in the end, it is really the peer pressure of the other 13-yr-old boys in the community that will determine what any individual boy chooses to do, much as it will be for the rest of their lives.
First of all you don't have to Lein for your bar mitzvah but I think the choice so it show your becoming of a man And it is very amazing that you get to read out of the torahh correctly at only 13
I once heard from Rabbi Nissan Kaplan that he did all mitzvos before then except for teaching torah to the masses. this is why he gives a drasha on the bar mitzva.
maybe the torah reading is similar idea. and also, maybe, to put him in the spot light and give him some honor, like a chatan to make the occasion a great simcha for him