Story of her life, though! Kaylee is also a vet, having been a mechanical engineer. She decided not to re-enlist when the military started using her talents towards unethical projects, which may have been leaked or contracted or sold to become Vault-Tec experiments. Kaylee traded those horrors for domestic "bliss", which was never something she'd wanted before, hoping to forget it all. She never really opened up to Nate, because why spoil his... awe. You get me. <3 What transpired in the Vault triggered it all back to life, and compounded with the grief of losing her loved ones. She's still in shock, bumbling around and helping whoever she can, but waaaayyyyyy underneath, Kaylee is very, very angry.