You want people to like you / want to be in charge of people / think you're a "leader"
You think you're smarter than everyone else
You think you have some technical "secret sauce" that no one else has and that this has intrinsic value
You think your PhD thesis is a product
Mummy and Daddy are giving you a seed round because they want to get you out of the house
You live in LA or New York and you're jealous of San Francisco
You heard that $fad (Big Data/IoT/Adtech/Fintech/whatever) was big
*handwave* machine learning *handwave*
You want to write code and not talk to people
You want to change the world
Good Reasons to Start A Company
You know someone with purchasing power at a big company or government who has a problem you can solve and who is willing to pay you real money to solve it. Building the product is essentially doing consulting for them.
You know someone on the CorpDev team at a big company who says something like "we'd really like to buy a company like X but there just aren't any we can buy"
You see a market niche that either nobody else sees or everybody else undervalues (eg: because you have experience that's unusual among the sort of people that start companies), and you can build something yourself that fills this niche. Note: don't look for a technical cofounder, become one. Even if you only learn a little of the tecnical stuff it will make finding people who know more than you much easier.
You made something as a hobby project that a lot of people are using, and at least some of these people have money
Someone (either a VC or the government) is willing to give you money to screw around, and you have nothing to lose. That someone should not be family, because you might not be on speaking terms with them in a few years.
You started doing consulting, and then you started hiring people because you were getting more work than you could do youreself, and one day you realized that you were running a company.