I think everyone interprets this song in a way according to what situation is currently tangled with their lives because it would apply to them. For me, I believe this is a girl waiting. Waiting for him to make up his mind, because they liked each other, but he just got up and ran away from it. He then comes back and she gives him a second chance, but he disappears again. She's begging him to stop giving her hope so easily, then snatching it away just as quickly. She realises it was probably all pointless but she still longs for the moments they shared together to come back. She clearly still has feelings for him and just wants to know 'Why' he's doing it all. Too many questions are still unaswered for her.
It makes me want to cry. This song is amazing.