A bar mitzva or bridegroom is usually (in congregations I'm familiar with, or, at least, Ashkenazic ones) given maftir. Why is this, considering that it's the lowliest of all the aliyos (e.g.)? (The question is made even stronger by the fact that a katan can get maftir even according to the common custom not to give him another aliya, so it seems a very odd choice to give a new gadol.)
Conceivably, another point might be the fact that maftir is more of a "public performance," so to speak, than the other aliyos. Nowadays most people who get an aliyah don't read their own portion, but the maftir nearly always does (except on the rare occasions where someone is called up for it and doesn't know how to read it properly).
I don't really know why, but here are two possible ideas:
Acharon, Acharon Chaviv (The last is the most beloved) - Rashi Bereshit 33:2
To give the same Aliya to every bar mitzvah boy (or bridegroom). Since the bar mitzvah boy (or the bridegroom) may be a Kohen or a Levi, he couldn't get any other Aliyah besides the Kohen, Levi, or Maftir Aliyah. If the bar mitzvah boy (or the bridegroom) was not a Kohen or Levi, he couldn't get the first two aliyas. That leaves only one Aliyah that could be uniformly given to everyone, The Maftir.
Many Chasidishe Shuls give Maftir to the Bar Mitzva boy the week prior to his actual Bar Mitzva. They can not give the Bar Mitzva boy any other Aliya as he is still a Katan. Those Chasidishe Shuls that give the Aliya after the Bar Mitzva give a regular Aliya, not Maftir.
I would imagine that this Minhag of giving a week prior has led to some giving it the week after. It also may be as they want to give the Brachos of the Haftora to the bar Mitzva boy.
I learned it as a combination of Menachem's point 1 (Rashi on beloved) and Gerson Gold's last sentence (the additional honor of the haftorah brachot).