Bring out the coffin, let the mourners come. (4)
Until this moment, the speaker has been making lots of commanding statements. This line introduces the beloved's death into the poem. Now we know why he's been calling for quiet, and we start to understand our guy a bit better.
Let aeroplanes circle moaning overheadScribbling on the sky the message He Is Dead. (5-6)
Here's the thing about death. People usually don't skywrite about it. Death and mourning are personal, and usually limited to a relatively small group of people. But the speaker's grief feels so huge, he wants everyone to pay attention.
I thought that love would last forever: I was wrong. (12)
The speaker's thoughts about life and love have been completely overturned by the man's death. He realizes that nothing, not even love, can withstand time. Death doesn't just snuff out life—it snuffs out love, too.