From interview with Alexis:

"Paul wanted to give Aaron Swartz, another YC founder, a birthday gift in November. More than anything else, Aaron wanted co-founder so Paul suggested the 'merger'. Merger is probably a bit hyperbolic for what actually happened, Aaron basically moved in with us and we made him a co-founder."

http://startupstories.com/2006/11/29/passion-for-your-users-will-come-back-alexis-ohanian-co-founder-of-reddit/