Thank Heaven, the sun has gone in, and I don't have to go out and enjoy it.

The line is a wry inversion of the conventional piety about fine weather. Instead of welcoming sunshine as an unquestioned good, the speaker feels social pressure to "go out and enjoy it"—as if pleasure were an obligation. By thanking heaven that the sun has disappeared, the remark satirizes compulsory cheerfulness and the moralizing idea that one ought to be outdoors, active, and appreciative. It also captures a distinctly modern, urban sensibility: comfort in interior life, books, talk, or solitude, and irritation at nature being used as a standard by which one’s temperament is judged. The humor depends on candor about laziness, introversion, or contrariness.