The Voice of Lews Therin

Early in the series Rand has access to some knowledge from his past life as Lews Therin Telamon (LTT), but he does not yet hear the voice. However, every time he catches himself knowing something or remembering something, he freaks out a bit thinking that he must be going mad, and he refuses to accept it as a part of him. His mind can't wrap itself around the fact that he is LTT reborn and that he can remember these things - because that is something that would be hard for anyone to grasp. Not to mention that he just doesn't want to accept that he is the same man that killed everyone he loved and broke the world.

As he channels more his insanity starts to develop into a voice and personality (which are in fact based on Rand's memories of his past life as LTT) so that Rand can still see himself as a separate person from LTT. This is because he doesn't want to be LTT - he's developing multiple personality disorder from his madness. As he suffers torture in the box during his captivity in LoC the voice gets much worse and he starts talking to it more - this supports that he keeps using it as a coping mechanism and his condition progresses.

Eventually he sees this personality as so real that he sees LTT as actually being able to seize and weave saidin in KoD Ch. 19. The weaves Rand uses while thinking it is LTT in control are simply weaves coming from the memories of LTT that Rand has. This is the same as he did early in the series (before the voice) when he remembered weaves out of nowhere and used them but now he uses his coping mechanism of his "LTT" personality so that he doesn't have to admit he is the same man (and also because at this point he really is mad from the taint on saidin).

As long as he had the LTT voice he could see himself as a separate person from LTT, but in tGS Ch. 50 “Veins of Gold” he finally confronts the fact that he is the same man. He remembers the day that he killed his family, seeing their bodies, seeing Ilyena. He even remembers killing himself. And because he finally accepts that that was him, not some other guy, that did those things, the voice finally goes away.

"For they were not two men, and never had been."

Quote of the Day:

“I seem to remember dying, Lews Therin murmured. I remember how I did it. He drew deeper still, and small pains grew in Rand’s temples.”

– KoD, Ch. 19