Sure, "The Sun Rising" is a love poem, but you'll notice the speaker never actually talks to his main squeeze. Instead, he's whining at the sun to leave him alone. Mostly, that's because he doesn't want the night to end. He doesn't want to rejoin the world. Hey, speaker, we never said love was easy. It's full of disappointments, so Shmoop's advice is to buck up.
Behind all his strutting, the speaker knows that the sun will rise, he'll have to leave his love and go back to his normal life.
At night, the speaker can pretend that he and his love will be together forever. When the sun starts to move across the sky, he's reminded of the possibility that it won't last, and that's where the real frustration comes in.