I feel like Josh went to Germany with the intent (whether he knew it in the moment or not) to eventually tell Donna how he felt but then Colin got in the way.
There's just something about the devastation on his face when Colin comes in and kisses Donna and he just completely stops what he's doing. I also think the conversation in the hall he has with Colin really turns him off what he was going to do. The line about "leading her on" makes him contemplate what happens when they go home.
I mean I could be wrong but it just makes sense for why they brought Colin in, like Josh says, why would he come all the way from Gaza. I get the flights like 4hrs but that's 4hrs for someone you only spent a few days with.
Maybe I'm just thinking into it to much, but it just makes sense. It explains (for me anyway) why things sort of come to a head in season 6 because I think Donna thought that's why he was there as well. I get they are close but for him to just completely drop everything and run to Germany is a massive deal for a man in his position. Also Leo giving him the speech before he leaves kinda confirms my thoughts as well. He know's how Josh feels about Donna but Josh choosing her over the White House really cements what he knows/suspects and aids in further confirming it for the audience.