Rand stated that Lews Therin was arrogant. Talking of Demondred if only he gave praise instead of mocking amusement. Raised him up instead of constant competition.

Another clue was how he talked of how things would have been much different if Lews Therin was raised the same way Rand was. Had the same people and support that Rand did. Rand was a humble farm boy. Lews Therin was not. That's difference was inevitably Lews Therins downfall.

It was Lews Therins ego that wouldn't allow him to let go of Illainas murder. "Why do we do this over and over again?" It was Rands humbleness that came up with "because she lives again." "We get a second chance" that's when Lews Therin let go and they became one.

Lews Therins worst enemy was himself.