What's a fundamental life lesson that people should know, but they probably don't?

This was inspired by a thread in r/AskTeenGirls. If you're over 18, don't bother going there, it's not worth your time.<br><br>You can share anything, it doesn't matter what it is, from "you should get over the mentality that you need to finish a whole meal in one sitting" to "when someone says 'don't throw lit matches in the trash', that's not the same as saying 'don't throw matches in the trash'". I'd like to hear your thoughts.<br><br>On-topic example: You can replace the brake pads on your own car for $20 and an hour if you know what you're doing, but a professional will charge you $100 for fifteen minutes worth of work.