Comments were implemented just six months after reddit's birth (December 12, 2005), and have since been an integral part of the site.
[Motivation for why comments were added]
A recurring joke appears in one of the very first comments, that "Reddit is turning into Digg." This comment has been brought up in most large reddit updates to show that this negative opinion of the future of reddit due to changes is not new. Digg, being a more popular vote and community driven social media site, was often seen as a rival site. Since Digg's version 4 update in 2010, reddit surpassed Digg in popularity and the joke and/or observation that reddit is turning into Digg has since died down, but still persists. [clean this paragraph up]
The very first comment on reddit was probably this by the user charlieb.