Picture this: it’s month 19 of your startup journey. You’ve finally got traction, users are pouring in, and the term sheet you’ve been dreaming about lands in your inbox.
Then your cofounder walks in and says the four words that instantly kill everything you’ve built: “I want to leave.”
Suddenly the cap table, the product roadmap, the investor updates—none of it matters. Your company just died of a broken heart before it ever got the chance to fail for the “normal” reasons.
I’ve watched it happen more times than I can count. And I’m not alone.
The Silent Killer No One Talks About (Until It’s Too Late)
Forget running out of cash. Forget product-market fit struggles. According to some of the sharpest pre-seed investors in Europe, the real number one reason startups collapse in the first 18–24 months is shockingly simple: the founders fall out.
They stop getting along. The shared vision cracks. One person wants to grind for five more years, the other wants to cash out or just… breathe again. Sometimes it’s ego, sometimes it’s exhaustion, sometimes it’s the slow realization that you actually never knew each other that well to begin with.
And when that relationship breaks, the company breaks with it—fast.
Why Cofounder Chemistry Is Literally Everything at the Beginning
When you have zero revenue, zero users, and sometimes barely an MVP, what exactly are investors betting on?
You. Both of you. The weird, messy, human bond that has to survive 80-hour weeks, constant rejection, and the kind of pressure that would crush most marriages.
One prominent pre-seed fund that just raised nearly $90 million says they make about 80% of their investment decision based purely on the founders—before there’s even a real product to look at.
“If the founders fall out, nothing else matters. Game over.”
Harsh? Yes. Also completely true.
The Questions Investors Are Secretly Asking (And You Should Ask Yourself)
Next time you’re raising money, know that the really good investors aren’t just listening to your pitch. They’re running a full background check on your relationship.
Here are some of the things they want to know—things most founders have never even thought to discuss:
- How long have you actually known each other? (Six months of hackathons doesn’t count.)
- Have you ever gone through something genuinely hard together—lost a job, a family crisis, a failed project?
- If I interview you separately for an hour, will your stories about each other match?
- Can you name your cofounder’s three biggest weaknesses without hesitating—and do they agree?
- Would you trust this person with your life savings? Your dog? Your darkest secret?
One investor I know literally pulls up the famous 36 Questions to Fall in Love from the New York Times and thinks: “If these two couldn’t answer most of them about each other, I’m out.”
That’s the bar now.
Childhood Friends vs. “We Met at a Startup Event”
There’s a reason so many of the unicorn founding teams you admire were friends long before they were cofounders.
Think about the duos that actually made it big. Often they’ve known each other for a decade or more. They’ve seen each other at their worst. They’ve already survived a thousand tiny arguments about money, credit, and whose turn it is to take out the trash.
Compare that to the classic “I needed a technical cofounder so I posted on LinkedIn” story. How deep can that bond really go in six months?
In my experience, the teams that look perfect on paper—ex-Google engineer + ex-McKinsey hustler—sometimes explode the fastest because they never built real trust when the stakes were low.
The Four Traits That Actually Predict Survival
Beyond the relationship itself, top early-stage investors look for four recurring traits that separate the teams that make it from the ones that implode.
- Domain obsession – One (or both) of you has been thinking about this exact problem for years, sometimes decades. It’s not a “good idea you had last quarter.”
- Relentless grit – Evidence you’ve been unreasonably persistent about something in your life. Elite athlete, chess master, built a business as a teenager—something that proves you don’t quit when it sucks.
- Growth mindset on steroids – You genuinely believe skills are built, not born. Feedback is oxygen.
- Complementary superpowers – Fast thinker + deliberate thinker. Charismatic recruiter + deep technologist. You finish each other’s… sentences, yes, but more importantly, you cover each other’s blind spots.
When one investor met the team behind a now-billion-dollar AI company, two things hit him immediately: they’d been friends since childhood and they were completely obsessed with voice synthesis for years before starting the company. He wrote the check after a single meeting.
Solo Founders: The Ultimate Red Flag?
Paul Graham once wrote something that still stings: a solo founder is basically a vote of no confidence in your own idea. If it was that good, wouldn’t at least one of your smart friends have joined you?
Harsh, but statistically brutal. The data backs it up—solo-founded companies have a fraction of the survival rate.
Startups magnify every crack in a relationship the way a pressure cooker magnifies flavor. If there’s even a tiny fissure, it will become a canyon.
How to Stress-Test Your Own Cofounder Relationship Before It’s Too Late
Want to know if your founding team will survive the apocalypse? Try these uncomfortable exercises (yes, actually do them):
- Write down independently what equity split feels fair. Compare. Discuss for two hours straight.
- Role-play the scenario where one of you gets a $500k job offer tomorrow. What happens?
- Take a weekend trip together—no laptops, no escape. Just the two of you and whatever comes up.
- Have the “worst-case scenario” talk: What if we’re broke in 18 months and fighting with investors? Who’s still showing up?
- Get brutally honest feedback from people who knew you both before the startup. What do they really think?
If any of those conversations feel impossible, congratulations—you just saved yourself three years of pain.
The Bottom Line Nobody Wants to Hear
Building a startup with a cofounder is the closest thing most of us will ever experience to an arranged marriage on hard mode.
You’re going to see each other more than your actual partners. You’re going to fight about money, credit, and vision on a weekly basis. You’re going to be exhausted, broke, and terrified together.
If you wouldn’t marry the person, maybe—just maybe—don’t start a company with them.
Because in the end, the product can pivot. The market can shift. Investors can be replaced.
But if the two of you break?
Nothing else matters.
And that, my friends, is the real reason most startups don’t make it past year two.
