Take a good look at what needs to be accomplished to build and scale your business
 Identify critical missing pieces you do not have expertise with or do not want to do
 Estimate how hard/easy it would be for you to manage these pieces yourself, with some hired help
 Weigh pros/cons of possible dilution / distraction / misfit vs. likelihood of acceleration / risk reduction / strength in numbers
 Make up your mind on what is the right way to go and GO FOR IT!
IMHO, saying "you always need a co-founder" is a cargo-cult-like mindset. There are plenty of successes and failures doing it with or without one.
The only thing that ACTUALLY matters is whether you can put together something that will make money. Whether or not you had any co-founders along the way is about as casually related as whether building an airstrip attracts cargo planes (http://en.wikipedia.org/wiki/Cargo_cult)
Just determine what you actually need and do not get distracted by people trying to mold you into their perception of the "ideal".
However, having made progress on something is a good way to attract a cofounder for something else later.