There are literally thousands of distro's available of which some of the most popular are very user friendly, have a great community, are stable and secure and have tons of software available in their repositories. So why would you choose a distro that is way harder to install and maintain?

First of all, nostalgia. Gentoo was my first distro back in 2002 when I started my own web company. I learned practically 80% of what I know about Linux by fooling around with Gentoo. It hooked me and it never let go.

Second, absolute freedom! I dare to claim that there is no distro out there that gives you more control over your system than Gentoo does. Gentoo uses a nifty portage system with use flags to control how your software is compiled. For example if I want to install Nikola I can choose from the following use flags: assets, charts, hyphenation, ipython, jinja, server, watchdog, webmedia. If I choose charts, approximately 25 python libs are pulled in and even some libs that belong to X11. If I choose ipython, about 85 python libs extra are pulled in. I don't use both, so my Nikola is lean and mean.

Third, security. By compiling your software with just what you need build in, you reduce the amount of attack vectors. For example, if you don't trust openssl anymore and would rather use libressl instead, you can fix that with use flags, system wide, or on a per package basis. I compiled git without cgi support, so gitweb is simply not available on my machine.

last but not least, the community. As I told you, I've learned a lot about Linux by using Gentoo. That wouldn't have been possible without the Gentoo forums. There you'll find people with absolutely the deepest knowledge about Linux ever.