The light is a representation of his power over her. Every time he touches her (or something of hers) the light goes off, meaning that he is in power and in control of the situation (the light). She is pissy because she wants the lights on, she wants to expose the secret relationship. At the end the light transfers over to him, thus making it his burden now (she leaves him).
http://www.youtube.com/watch?v=7FxqBssNfSw
The light is a representation of his power over her. Every time he touches her (or something of hers) the light goes off, meaning that he is in power and in control of the situation (the light). She is pissy because she wants the lights on, she wants to expose the secret relationship. At the end the light transfers over to him, thus making it his burden now (she leaves him).