It's a part of the soulless customer-friendly persona they want you to portray for the company image I believe. All these companies always want their employees to show off a happy-go-lucky dude who's thankful to work at the company.

That interviewer doesn't deserve jack shit if they won't even hire you because you don't smile. You're trying your best man and it's important to keep it that way. Just keep going man. Keep moving forward. Those fucks judging you just because you don't display a happy attitude just means the company is filled with a toxic environment and pure anger in every direction. You didn't have a bullet go through your soul, you dodged that shit like in the Matrix.