In my regional language, there's a very good idiom which literally means...
'X' was already my favorite dish and a doctor advised me to be on 'X' (this 'X' diet/dish)
Now the context.
This idiom is used when someone wishes/likes something and on top of that, legally/officially he's asked to do that!
"Jack is not a dedicated person. He never likes to work."
"I know...but do you know he's been offered a field work."
"What? a field work? Oh my god! So now, he'll simply roam here and there!"
Yeah... [Idiom here] meaning he already love wasting time by not doing work and now he's offered a field work where no one will keep a watch on him. At least, in office, he was doing something as everybody watched.
To make it clearer -
It does not matter whether what you wanted/liked/doing is a bad thing or good. The idiom has nothing to do with goodness or badness. It's just what you already wanted/doing/liked, it's (by luck?) told/permitted/asked to do -all official and legal!