When an employer rejects you after an interview, there is unfortunately no incentive for them to explain to you why you were not hired. In fact, there are actually some incentives for them to offer no explanation at all. They don't want to run the risk of citing any reason that could open them up to a lawsuit, and they don't want to enter into an argument with you either.
As a result, it's pointless to ask why you didn't get hired. They probably aren't going to respond or give you the real reason anyway, and this question will sound confrontational, no matter how delicately you choose your words. Instead, you should ask what you can do to enhance your candidacy for future opportunities.
It's wise to ask for feedback in this case by sending an email. This way, the hiring person has some time to choose if/how to respond to you. It's also easier for them to respond to an email than if you send a letter and expect them to take the initiative to send you a written/email response in return. Here is an example of what you could write in an email to the hiring person if you are not chosen for a job:
Dear Mr. Interviewer,
While I'm disappointed that you have chosen another candidate for this position, I appreciate the opportunity to interview with you.
Since I'm always looking for ways to grow professionally, do you have any suggestions on what I can do to enhance my candidacy for future opportunities _________ (i.e. in this field, with your organization, etc.)?
I respect your opinion and appreciate any feedback you might have.
Thank you again for your time and consideration.
This approach is much more effective than the "why didn't you hire me?" approach for a few reasons. First of all, it puts the emphasis on you rather than the employer and implies that you respect their decision (even if you don't understand it). Secondly, this approach demonstrates a desire to improve yourself, a trait that every employer respects.
While the employer might not offer a clear answer (or any answer at all), it's possible you may gain valuable feedback on how to improve your chances for employment with their organization or with others. If nothing else, you position yourself as a proactive, self-motivated candidate who understands the importance of professional development. By remaining professional and courteous, you also keep the door open for future opportunities.
One of the success stories featured in my new book used this exact technique to land his dream job right out of college. After being rejected for a position with Polaris Industries, Austin Moyer sent an email to every person he knew within the company to ask what he could do to enhance his candidacy for the future.
The result? When another position became available one week later, the organization brought him back in for another interview, and he ultimately got hired!
In summary, if you get rejected for a job interview, don't get bitter. Instead, ask how you can get better. You might be pleasantly surprised what can happen with this approach.
Pete Leibman is the Founder of Dream Job Academy and the Author of I Got My Dream Job and So Can You: 7 Steps to Creating Your Ideal Career After College. His work has been featured on Fox, CBS, and CNN, and he's been invited to speak at some of the world's best colleges, including Stanford University and Johns Hopkins University.