I come from a generation where you didn't retire from Scotland, Scotland retired you.
So, the entire notion of calling time on your country is a bit alien to me. However, people do change their minds, and they are entitled to do so.
Hopefully, the family situation which Kris cited as being behind him stepping away from Scotland has been resolved or improved sufficiently to convince him he can now make himself available to Gordon Strachan again.
If he does want back, he knows who to call, and I'm sure Gordon would be ready to welcome his return.