It's narratively convenient to be able to point to a single event and say "this is why the character is this way about a thing", and sometimes, that's good.
Other times, it makes me want to throw something at whatever screen I'm looking at, because plenty of things are the result of gradual circumstances and feelings over time rather than a singular event, and sometimes, there's no reason beyond "that's just how they are".
(EDIT: Okay, I did actually say "no reason beyond", but I didn't mean that literally. There are reasons for things. They're just not always as obvious as "these specific things happened in a person's life and now they feel this way.")
Various phobias, for example. Phobias are irrational fears people can have without needing some traumatic event to cause them. Those certainly won't help, but you don't need to explain why someone has a major fear of spiders. For all intents and purposes, a LOT of people are really afraid of spiders by default, and the majority don't have some "well, one day, back when I thought spiders were lovely, BUT THEN...." story to explain it.
Sorry, got on a bit of a tangent. That's one of my gears that gets grinded, you see.