When they were recording Joshua Tree Bono didn't make it to Ali's birthday, so Bono wrote this song. Bono said this in an interview when they were shooting the video for the 1998 release of the song and this makes perfect sense when you see the video.
Jonas Steverud (Maintainer of U2MoL) (27:th of November 1998)
I feel that The Sweetest Thing also additionally speaks to the volatility and frailty of love. At any moment what seemed to be solid can change Eternal fire, she turned me to straw. As to frailty, You can sew it up/But you still see the tear. Another related theme that permeates the song is that love can be difficult on both people My love she throws me like a rubber ball...But she won't catch me, or break my fall and I wanted to run, she made me crawl.
Hawkwing fivewoodteleport.com ( 4:th of January 1999)