Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
So, I have been asked this question many times. And of course the assumption is that I am some sort of a masochist who enjoys wasting time on compiling packages. However, that is not entirely true. There are some other reasons why, after trying Ubuntu (started linux with it), Gentoo and then Arch I settled on Gentoo.
The main reasons in a nutshell would be:
1. Rolling Release - Arch does the same thing, while you could achieve the same if you keep using the development releases of Ubuntu. (However, once a feature freeze sets in, you have a wait of couple of months before new packages move into the repos). Arch, in fact, is more bleeding edge that gentoo. However, that is an advantage and a disadvantage as well. While it allows you to have the most bleeding edge system, minor breakdowns are expected. The same can be said for Gentoo - however, with the option of hard masking (not normal masking) a saner solution exists.