The One Big Happy in Thursday’s comics feed:
From OED3 (Nov. 2010), under labour ‘task’:
labour of love a task undertaken either for love of the work itself or out of love for a person, cause, etc.; work of this nature.
The fixed expression — which Ruthie has presumably not heard before and so has eggcornishly reshaped — is Biblical in origin and has been used allusively in names of books, songs, films, and the like.