Ever since the last episode of season 7 of Game of Thrones came out, one question that's been getting everyone in a tizzy is why Tyrion was staring with such concerned bemusement at Jon and Dany doing the dirty. There were a whole lot of theories doing the rounds, but now, people from the show itself have weighed in on that enigmatic expression.

Source: Vanityfair

According to a Peter Dinklage interview, "Jon and Dany's incestuous relationship is dangerous for everyone involved. I'm sure it's good for both of them - in that moment - but you don't even get the sense of how beautiful it should be. It should be, but it's not. This is Game of Thrones, there's a long history of romance not ending well on this show."

Source: Esquire

Some people seem to think that Tyrion's actually fallen in love with Daenerys and is jealous of Jon, while others believe the worried expression is because of some unseen pact he's made with Cersei. Well, they cleared that up too.

In an interview with The Daily Beast, Jeremy Podeswa, director of The Dragon and the Wolf said, 

"I think there’s jealousy, but it’s maybe not romantic jealousy, in the way that it is for Jorah, for example. I think that for Tyrion, it’s more complicated. I think he has a very special relationship with Dany and he really believes in her as a true leader and has invested a lot in her. I think for him, with Jon and Dany getting together, this represents a possible undermining of his position with her and also a monkey wrench thrown into what the master plan really is meant to be around this entire alliance."
Source: Joe

As for the bits left unseen in the Cersei-Tyrion meeting, he said, 

"I think the assumption is that there definitely was more conversation after that, which leads to her coming back to the Dragonpit and agreeing to be part of this alliance—we think, at that point. (Laughs.) So yes, there was definitely more conversation after that. But that’s all gravy. The essence of it is where it gets to at the end of that scene. I think, for me, that’s one of the great moments for both those actors. I literally had to take a breath. The first time they played [the pregnancy reveal], I was like, (gasp). It was a very powerful moment."
Source: Aminoapps

On the whole though, Podeswa seems to think that it was mostly just concern for how the Jon-Dany union would affect her decision-making abilities in regard to Tyrion.

"The way I see it is Tyrion is a bit of a strategist—not just a bit of a strategist, he is a major strategist—and I think now, he can’t see where this is gonna go and that’s very difficult for someone who is always thinking three steps ahead. The consequences of Dany and Jon getting together are completely unknown. Is she gonna make decisions now based on this new relationship? Is she gonna be able to separate her personal [interests] from the interests of the greater group? What is this going to foretell for the alliance and what they’re all meant to do as a united front? So I think the worry for him is that now, everything is up for grabs. We don’t really know who’s going to side with who, what’s gonna happen at the end of the day, and which alliances are going to be the strongest"

So there you have it, the inside scoop from the people most in the loop. I don't think we're going to get any more information than that for the next two years, so cherish it!