I wouldn't say that they were "addictive" as much as they're a simple design that's easy to wrap your head around. That makes them a great choice for folks that like to shoot without having a lot of extra "stuff" on their guns like manual safeties and extra parts that make them hard to detail strip in order to maintain them.
You can totally take a Glock apart down to component parts with a 3/32" pin punch, can't do that with any other gun that I know of.
Plus, the aftermarket has helped out by offering a lot of great parts so you can upgrade or customize your Glock to suite your needs. Only other handgun I can think of with the same aftermarket is the good ol' 1911a1. That puts the Glock in some pretty good company IMHO.