No way... that's crazy...
You know what they say, wealth always skips a generation.
by TurncoatWell yeah, but what happens externally affects how they are internally. Whatever happens to them first profoundly would likely lack a frame of comparison.
As spite put it, "there's so much to account for."
You asked: "How much do you think someone's first loss shapes a person into who they'll eventually become?"
I don't think it's possible to arrive at a rule or a generalization for how a first loss will shape a person beyond that how that a "loss" is interpreted determines its effect.
I learned a very interesting and fundamental difference between mainstream American culture and Islamic extremist culture while following the Gaza conflict. Here in the States, when someone dies, it's often the case that the event is a melancholic affair. We've all seen the scene of a loved one's casket being lowered into the ground, while family and friends dressed in black wipe their tears away during their "final goodbye." Well in Gaza, when a significant other died there was certainly some sadness, but this sadness was tempered by religious ecstasy. Fathers would speak of how proud they were of their sons for becoming martyrs for the cause! When the Palestinians found dead bodies, often they would scream "Allah is great!" repeatedly. Think about the effect that culture alone would have on how a person experiences their first loss.
I think my first encounters with death being pets and people I barely knew (church people) helped me to believe that death happens and it's useless to dwell very much. If it ever made me upset, I decided it was best to figure out why and move past it. When the boy died in a crash in high school, two of his friends in the car survived (the driver with no serious injuries and another passenger in a coma) it bothered me because I never really considered I'd die in a wreck at 15. Of course, I wasn't letting cripples drive me places, but that's not a story for today. Losing pets upset me because it was so difficult to talk my parents into them in the first place. They're replaceable, though sometimes expensive (replacing a definitively good dog or fancy cold blooded critter). I realized the same is true of people, in a sense. Maybe no one can replace that individual for me, but there are always more humans.