Because king Garon stole away for memories of your childhood, sent you on a suicide mission with Hans knowing he'd try and kill you, and then gave you a cursed sword, knowing you'd go to Hoshido and meet sweet sweet Marisha Ray who you would inadvertently kill ( along with yourself and several dozen innocent people). All the while you realize that Nohr resurrects the dead to send in and slaughter innocent Hoshido people. Even Camilla is like "Wow, everything you said made sense, I'm so sorry for trying to hurt you. I'm going to defect." Until Otouto was like "FUCK THAT TITTY LADY." Literally, the only person who doesn't end up deciding that Nohr is full of shit is Xander and even then… stuff happens.

(Or Gamestop was out of Conquest and I had to settle with the last Birthright)

Well to be fair, at least you older siblings in Hoshido planned to raid Nohr and rescue you.
They just… Didn't think they'd take you in and love you…