Only when Eric told Seraphine about it did she realize that she had actually done something that she kept avoiding all this time.
She swore that she would never get close to anyone again. Though there was the probability that he was not influenced by her power as a succubus, considering that he was blind and he had certain circumstances that made his kind treatment to her understandable, she did not want to take another shot at that cycle of abandonment and estrangement.
Aside from that, it would not be good for Eric to stay with Seraphine.
First of all, she can't provide for both of them. She is even barely surviving on her own. They'll only bring each other down - she'll only bring him down, rather. The palace is where he truly belongs and not in the streets. There, he can unleash his hidden potentials, get all the training and studying he needs to develop, earn power for himself, and most of all, have a better chance of surviving than her.