Xenophobia is the instinctive initial reaction that all animals have to survive,
the reaction that makes us briefly react to everything unknown with fear, like a baby crying when introduced to the concept of balloons.
The next instant step of instinct once the unknown is seen not immediately lethal is curiosity, to examine what was feared closer and identifying what it actually is to often find the unknown once known to be something liked.
This second phrase of instinct is why we have things like scary movies and skydiving.
Some don't push past the initial fear and when the fear is allowed to remain, it putrefies into irrational aversion: hate of the unknown other causing harm both to self and others.
Often this is due to an environment encouraging social cowardice or hinder the perceptions that would identify the unknown as beneficial; sometimes it is just mental laziness.