Why was the subjunctive used here?
These lines are form the lyrics of the song Recuerdos de amor by Victor Y Leo. Please help me understand why the subjunctive was used in the line written in bold letters:
Tienes razón
Que las palabras no resuelven muchas cosas,
Pero es lo mejor, debemos conversar.
Al final, nuestro caso no es distinto de otros casos que acabaron mal.
Y debo confesar que aunque he sufrido ya,
Las noches que no estés, sufriré aún más.
Thanks in advance!!! 
2 Answers
He's speaking about the nights in the future when she won't be present. If he was speaking in general about the nights when she isn't here, the indicative would be used:
Las noches que no estás, sufro aún más.
Additionally, if he were referring to specific nights in the future of which he had knowledge of her absence, the subjunctive wouldn't be used:
Vas a la boda de tu hermana este martes y miércoles. Las noches que no estarás, sufriré aún más.
The subjunctive is used here because the singer does not know when his 'amor' will be with him. So that is subjective.
