But it's different when you're actually PLAYING for and representing a country.
Fine.. I'll leave it alone. I just don't like it, and I'm not sure if I can follow him anymore now that he's turned his back on Canada.
No it absolutely isn't. Where you live physically means nothing - this is the 21st century. He can stay wired to Canada, his friends, family etc... with absolutely no problem. His girlfriend is Serbian - it makes total sense he lives there. And for a tennis player with a ranking that means he needs to play challenger events, Europe is the most cost effective and opportunity rich environment for a player - I think its great actually, and may help him mentally to relaunch his career.
You mentioned Milos by the way "not turning his back on Canada" - for all intents and purposes, milos lives in Spain as much as Canada these days, and again good for him.