How to validate ideas without deceiving yourself

We've all been there, you and your team mates have what you think is a great idea. Maybe its an awesome new gesture, or a feature you think the customers are going to love. You do everything you're meant to, you interview existing customers and potential users, you build and test multiple iterations of the idea. Finally just when you think its going to be a massive hit, you launch and ..... its a flop, customers don't use it, new user feedback glances over it and later even your friend tells you 'it kinda doesn't work'. 

But what happened? You followed the process, did everything by the book, right? 

Wrong. Let me tell you why and how you can become a better interviewer.