It is also important for the company to understand exactly why the person is leaving. If the person is leaving because he doesn't like the team that has been put together, then it is crucial that the founders know that.
Chances are the person that is leaving knows something the founders don't. Squandering the opportunity to find out exactly why they left would be a huge mistake.
(By the way, 99% of the time when an employee voluntarily leaves, it is because of the startup. If they are going to "pursue another opportunity", it means they have fallen out of love with the startup and something may be very wrong)
Dharmesh, although the advice throughout this post is all valid, isn't your most important observation in the first paragraph. Sometimes being "lucky" is better than being good. Every founder should strive to hire "extremely competent individuals". Couldn't you have stumbled upon the true secret by hiring people that were "extremely loyal to you"? Expertise is important. Enjoying the "work" is important. Passion for the mission helps. But, if the employee believes in, enjoys spending time with, respects, etc. the founder, this may prove to be the strongest glue of all that uses the personal relationship to keep the professional relationship together.
86% of people leave all jobs, because of thier boss. Do you really have to ask to know that?
As the person leaving, it is essential that you not run away, and it is essential that you not walk away with a checklist of things to avoid. If you do the latter, fate will surprise you and you will meet your nemisis again. If you do the former, it only takes you downhill to a series of worse situations. Breath. Think about life goals. Boost your self esteem, and then look. Get a raise. Get more responsibility.
When ask in a job interview, by an interviewer trying to ask behavioral questions, if you are a job hopper, tell them that that's a trait-based question indicative of companies not people. Then, ask them what they would do to make sure you don't leave.
Remember what it was like to be an owner. Remember what it was like to build a company from scratch. You might find institutional employers impossible to work for, so find another startup, and starup again.