Dolly is alone on a quiet street reflecting on why she is doing all of this. Her dialogue makes it all clear:

(to Heaven)
Ephriam--let me go. It's been long enough, Ephriam. Every day for all these years, I've locked the door, I've put out the cat and made myself a little rum toddy. And I said a little prayer thanking God that I was independent--that no one else's life was mixed up mine. Then one night, an oak leaf fell out of my Bible. I put it there the day you asked me to marry you, Ephriam. A perfectly good oak leaf--but without color and without life. And I realized that I was like that leaf... for years I had not shed one tear nor had I been filled with the hope that everything would turn out well. And so, I have decided to rejoin the human race. I'm going to marry Horace Vandergelder--and Ephriam, I want you to give me away."