We've all said something along these lines to our alarm clocks. We can picture the speaker waking and getting upset that he'll have to get up and at 'em. And his frustration is made even more emphatic by the trochee that starts off the line.
Why dost thou thus, Through windows, and through curtains call on us? (2-3)
This is really just a development of the first line, but it specifically accuses the son of deliberately taking away his happiness. Hey, buddy, it's not like the sun meant it.
Thou, sun, art half as happy as we, In that the world's contracted thus. (25-26)
Sure, this quote seems like it's happy boasting, but remember that the speaker is now trying to convince the sun to linger around and let them stay in bed forever. The whole argument is getting more desperate—he knows no matter what he says, it's only a matter of time until they will have to get up and face the real world—with clothes on, hopefully.