It probably has something to do with pg frequently reminding us that "single founder" is a top reason that startups fail[1], and saying his top advice for single founders is "find a co-founder."
And perhaps trying to convince him to take single founders seriously, or at least to engage with and respond to these counter-arguments so we can understand his position better.
[1]: http://www.paulgraham.com/startupmistakes.html