Ikaris and Sersi deeply loved each other, happily living with each other for over five thousand years.
As the Celestial emerges from the planet, the planet and all of the living creatures on it will get destroyed in the process.
Phastos realized that the Deviants were focused their attacks on him and Ajak, and Ajak connected it to the Deviants faced by and in Heiyan-Kyo and by and in Korea.
This act deeply moved her that would be capable of doing such act and their actions to save the universe.
According to our intel, Richard Madden's Ikaris will be responsible for the death of Salma Hayek's Ajak, the erstwhile leader of the Eternals, presumably in some sort of power grab tied to his.
While Arishem is the primary villain in Eternals, Ikaris' betrayal of his teammates hits the hardest.