Hi, there's no reason to take your US passport in Europe. I did recently read about a UK person whose flight back to the UK from Central America was unexpectedly re routed via US, and because the passenger didn't have an ESTA, the airline then refused passage. So I'm think that if I ever have to go somewhere that might get re routed via USA, e.g. Canada, Caribbean, Central/South America, I'd take the US passport along just in case.