When your co-founder leaves
2 weeks ago we went into a coffee shop and our co-founder told us what we had already seen coming: he's not as passionate about our product than we are, and that he wants to go back to school, finish his masters degree and pursue a career in a different industry.
Although this is sad, it's reasonable and fair: For a startup, you need 100% commitment, no doubts. Many startups fail because founders have different levels of commitment, which causes conflicts. We're still friends and enjoy hanging out with each other, it was just not the right idea for him.
And here's the positive thing I learned about this: we thought it might be a bad signal ("co-founder doesn't believe in the idea" stuff), but it has actually been the opposite. We were very transparent about it, and the fact that we're still doing this and how we handled it, was a clear sign how determined we are.
It's not "ooh, one left," it's rather "two stayed, and they mean it."