Happiness and productivity
Happiness, in all things not just the [choice of programming] language, should be the number [one] goal and metric for everything in an early-stage startup. Happy engineers work smarter, longer, more efficient[ly], attract better candidates, and have a better quality of life. (A corollary is that if you’re already set on a language path, don’t hire anyone who isn’t thrilled with working in that language.)
Relatedly:
We only do well the things we like doing.