Well, Aiofe said that Scathach lies all the time.
But in my opinion, Scatty is really sad because the man she loved is dead. Moreover, Scatty had a role in Cuchulain's death. I'm not saying that it's her fault, but the fact remains that she is the one who fought Cuchulain to death.
She is happiest when she's alone because when she's in the company of humani, she remembers that man he once loved, and she can't bear the pain that he had caused.