The truth hurts, and yet the truth is the most important thing you need to hear when you’re building a career. Unlike a sponsor, who can advocate for you as you seek a promotion, a mentor gives you unvarnished advice. And it’s the truly valuable advice that you can’t get from your friends, or often even from your boss, that you need to know but no one wants to tell you.

So why do you need a mentor to tell you the truth (even if it hurts)? Well, if you’re like most ambitious people you’re are probably quick on your feet, have a higher than average IQ, and can be defensive when faced with criticism. This is the type of behavior that discourages people from giving you feedback. Often successful people are blind to their faults. They can see them in others, but they are so busy charging ahead that they don’t allow themselves to see their own faults–after all it might slow them down! However, if you have a mentor who you know wants to see you succeed, you can actually grow from their feedback for the following reasons:

• A mentor can watch you in a meeting and coach you on your behavior and your tone.
• A mentor can listen to you whine and explain how you’re limiting yourself with your negative perspective .
• A mentor can challenge your lack of confidence and discuss why it’s important to be aggressive.
• A mentor can encourage you when you’re exhausted and on the verge of throwing in the towel.

In my experience, my mentors were the truthful cheerleaders–telling me when I was being obnoxious, unfair or weak, and pushing me ahead when I had self-doubt. I doubt I would even have had the opportunity to be a tech CEO, nevertheless a successful one, without my mentors Harvey and Bob–I am deeply grateful.

Read all answers to the MPW Insider question: Why is it important to have a mentor?

What qualities make a good (and bad) mentor? by Karen Tegan Padir, president of application development at Progress Software.

Why mentoring is unlike any other professional relationship by Jenni Luke, CEO of Step Up.

Why you don’t need a mentor to be successful by Beth Brooke-Marciniak, Global Vice Chair of Public Policy at Ernst & Young.

What qualities should you look for in a mentor? by Gay Gaddis, CEO and founder of T3.

4 things to consider before choosing a mentor by Camille Preston, founder of AIM Leadership.

The most important quality a mentor should have by Kathy Bloomgarden, CEO of Ruder Finn.

Why women are more likely to be mentors by Alyse Nelson, CEO and co-founder of Vital Voices Global Partnership.

3 reasons every employee needs a mentor by Sally Blount, Dean of Northwestern University’s Kellogg School of Management.

Why this AOL executive chooses her mentors — wisely by Allie Kline, CMO of AOL, Inc.