Co-founder dynamics are extremely important and extremely fragile.
You are choosing someone to go on long term journey, where you will no longer only be building something cool, but you'll be building a company, be responsible for hiring and, God forbid, firing people. You will be responsible for your employee's lives, and their families, you will be responsible for your investors time and effort to make all your promises come true.
All of this becomes extremely harder after you've been traveling alone for a long time (which can often be a few months, as startups are that intense). Sharing all this responsibilities with someone who has not been there from day one is challenging for both parties: can you trust them to be with you in the long run, do you agree in fundamental things, how do you deal with different company stakes and vesting periods? And, if you ever pivot (as many successful startups have[1]), how will all of these pressures come to live?
And it gets worse after you get investment (in the form of YC money, angel money, and progressively more) as your choice of co-founder will likely reflect on you, from your investor's idea. And it always add risk to the table, no matter how you put it.
All of this while still working and growing your startup.
Don't take this lightly. It may be harder to find a proper co-founder under these circumstances than founding a successful startup. But being able to have people you trust beside you building a company can be an invaluable thing.
[1] http://www.fastcompany.com/1836238/how-eric-ries-coined-pivo...