Because it is fun and I don't feel like filing Form 3794-G to have a plant on my desk or something.
I worked in the US Gov't as my first job and the amount of wasted time due to bureaucracy and red tape was overwhelming. I will admit comparing the government to big business isn't fair, but I can't imagine it is a far step.
Also I prefer working on scrappier teams where the work is either the entire project or a large chunk, rather than being responsible for one screen or something.
I have never worked in a large business but it always makes me shake my head when I hear a place like Pinterest or something having 2,000 engineers. Like how? What can 2,000 engineers do that 200 can't?
You don't need 200 dev-ops. You don't need 200 front-end engineers. It just baffles me why these places with one product need so many engineers. I am sure it is business people thinking nine women can make a baby in one month.
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I will admit that the whole equity thing tripped me up at first. Now I just want a large salary, keep expenses low and hope I can get a few years before "It's been a fun ride".
Also, for any junior startup people: No one is your friend. Absolutely be friendly and sociable but remember that it is a business at the end of the day.
Regardless of how many times the CxO says "We're a family!", you aren't. That same CxO will be forced to cut X% when times get rough. They may not like it but it is a business, not a family.
Dwell on the worst cases, keep your resume sharp and trust your gut rather than your heart. You may love life, but tides shift fast in startups (both good and bad).