If both people feel the same way about each other, and expect the same thing in return, that feeling and its effects are amplified. This is generally more true the more self-absorbed someone is.
You see this all the time. Someone is preoccupied with being slighted, and sees some sort of grimace or look of pain in the other, which confirms that the other is disgusted or being disrespectful in some way, without taking into account that the other has problems and preoccupations of their own.
So the first person, upon feeling slighted, reacts negatively in turn, and the other sees this, and the antipathy is established.
This process can be halted upon recognition of the humanity of the other, and not a mere object reflecting one's own ego.
