My co-founder didn't want to work on my next start-up idea with me because he was too tired.

I didn't want to work on his next start-up idea because I didn't understand the business model.

So I thought about finding a new co-founder.  Which made me think about what makes a good co-founder.  At the most basic level, they have to

• get along well with me
• have complimentary skills
• be ruthlessly authentic enough with me to hold me accountable

And it made me realise the relationship between co-founders is make-or-break for start-ups.  If co-founders don't get along, can't agree on things, or are too polite, the start-up is what suffers.

I didn't want to do a dance to find another co-founder I'd have to get used to spending 120 hours a week with, so I worked my hardest until I convinced my current co-founder to be my co-founder again in my next venture (I found out every single concern he had about doing another start-up with me over the course of a few weeks and allayed all of them).

Good things are rare and worth fighting for.