Personality in animals is primarily genetic (this is true for humans as well... well for many things, not everything, and you can successfully fight against your genetics), so if someone notices a parent is aggressive and breeds it, then typically the offspring will be more likely to be so. Note that a LOT of genes have an effect on personality, so it isn't necessarily something that will always prove out.

Anyway, if someone's genetic stock is based on a couple of snakes and they are aggressive then that explains the likelihood of their snakes being aggressive, hence someone saying one morph is more aggressive than another, its because their experience is based on a limited genetic stock.

Oh and I dont mean to offend people by saying that you're like your parents, its entirely possible that you're not (we all know genetics here, we know about hets and homs and how they are passed on), but you are genetically predisposed to a certain brain chemistry and hormone activity (genetics entirely keys for proteins production), your response to your brain's actions is partially experienced based, so you can react in a different way to how your mind would tell you to.