This is the A-Number-1 reason for having a cofounder. Inevitably, you're going to get burnt out, you're going to lose faith. It's at those moments that you talk to your co-founder and he tells you why you're out of you're fucking mind, that this thing you're building together is going to be amazing. And you get re-energized.

And two days later, you'll be doing the same for your co-founder :)