There's occasionally a sentiment in sport that you don't retire from your country, your country retires you.
In the world of football, where club tends to rule over country, that's something of an outdated notion. But, in the international-focused world of rugby union, that ideology has largely remained the norm.
You don't tend to see players publicly call time on their international careers to prolong their club future, as you do in football. It's just that, at some point or another, you're no longer picked.
