You need a co-founder if you want your startup to be successful

I don't agree at all.
You need to be smart and brave enough if you want your startup to be successful.
That's all.

You don't need a co-founder to succeed. There are many examples of single-founder startups succeeding. There are also many examples of startups failing because their co-founders couldn't agree on the direction of the company and ultimately had a falling out. There are also many examples of co-founders with no programming skills and no money trying to recruit a technical founder to build their dream and being disappointed that they can't find a rockstar engineer for 5% equity and no salary.

Co-founders can be especially troublesome when one or more of them can't afford to quit their day job, because they can't get much time to collaborate with each other, so they each take the business too far in the wrong direction, and/or their contributions are lopsided, potentially making the one doing most of the work resentful of the other(s). A solo founder doesn't have to worry about any of that.

It's hard for people rebellious enough to start a company to remain focused on the same vision and complement each other's work long enough to launch. It's even harder when you're not doing it full-time together. Having co-founders quite often means wasting each other's time in a battle of wills. Synergy is the exception, not the rule.

Made all the difference for me!

This has to be one of the biggest wildcards of startup success. You're more likely to be wildly successful with a co-founder. You're also introducing another risk of failure. So it's a trade-off.

I also dont agree with it at all. It creates sometimes a more mess than a solo entrepreneur.

Not necessarily but having people supporting you is important.

Honestly, awesome article. Love the bit about Steve Jobs saying "no", I have very much felt this as my partner and I focused our OKRs for this year and our quarterly goals.

Nice post! One thing I can add as a co-founder myself is; to try to find co-founder(s) who are competent in areas where you are weak. If you are building a web application and have great commercial expertise, try to find someone who has coding and software development skills.
I believe that solo founders can also be successful only when they have

1. prior startup experience
2. a good network of both investors and people likely to become their team players
3. success and failure stories&experience accumulated in years

Co founder is the best option to run and grow startup because it is very hard to run a business. But being emotional in the business is very bad do not make any other co founder which you think is the beneficial but you need to check is his/her skills will boost, improve the product how he/she will help in the business.

On the other hand if you make other co founder so you can enjoy your time it will be a major problem.

Select the person according to their skills and experience.

Disagree that your co-founder should be someone you know. Started a business with an old school friend and it was a disaster. Second time round hired a stranger as a co-founder and things are much better. Sometimes the emotional connection can be the problem just like in some cases it can be what works, but not in my case.

I was about to argue but then read the whole post and I agree. It's not just any co-founder you need it's someone who has specific qualities. When I launched my startup with my girlfriend at the time, we both had the determination but didn't have the expertise, so, of course, we failed. I'd argue though that solo founders can be successful if they have all the skills you mention but it's just a lot harder to do all that work by yourself.