Question about "The Fountainhead"?

I'm writing an essay on how the individual is portrayed in "The Fountainhead". Ayn Rand's theoretical aspect of Self-Interest suggests that everyone must strive for their own success and happiness without isting others or receiving help in order to achieve. Howard Roark obviously embodies this principle. I was just curious as to why Peter chose to marry Dominique when he loved Katie and how this reflects his weakness (which ultimately leads to his downfall)?