It goes up and down. The Euro launched at an exchange rate on $1.16 for €1. It then dropped at first, but rose again.10 years ago (=as far back as I can find a chart right now), it peaked at $1.60 for €1 (April 23, 2008 - it stayed there for a few months). It has been an almost continuous drop from that level, some ups and downs, but it is now at last exactly the same $1.16 that it launched at all those years ago.
So most of the drop is a correction from the dizzying heights it climbed to during the 2008 crisis of the US dollar. Some of it is probably related to the debt crisis in Greece, which never seems to completely end. Some is Brexit, I'm sure, but since Britain was never in the Euro, that effect is limited at best.
I think it is becoming more obvious to financial markets that the industrial base for the Euro is really rather small - Germany is the only really strong economy among the big countries. Spain and Italy rely heavily on parts of the country that are financially strong (Catalonia and northern Italy, respectively), France has structural issues that it doesn't seem to want to tackle, and the rest of us are smaller economies. Countries like the Netherlands or Sweden are actually doing really well right now, but we're small. The countries in central and eastern Europe are growing their economies and will be strong contributors over time, but they're not there yet.
At the same time, there are challenges ahead. The mood is very much against more integration right now, and nobody really wants to expand any further (there may be some expansions in the Balkans, but those countries are also small and financially weak), so what should the EU do?