I was around 13 years old when I first played New Vegas
My first playthrough, I killed the first NCR trooper I saw because I liked his service rifle and wanted it for myself.
Turned out the NCR didn't like it when I killed their soldiers, so they hated me. When they sent their hit squad after me, the feeling was mutual.
My first introduction to Caesar's Legion was Nipton, where they had burned the town to the ground and killed everyone inside. They had Powder Gangers on crosses, which I liked because I thought the Powder Gangers were lame and I thought crucifixion was neat.
When I met Vulpes Inculta, he had a very cool voice and disposition. The Legion's overall aesthetic had impressed me.
Then I came across my first NCR versus Legion fight and intervened on behalf of the Legion.
Fast forward to some time after meeting Yes Man. I met Caesar, who let me choose how Benny would die. I had him nailed to the cross.
It was at that point I knew I would fight for Caesar's Legion. Ave Caesar Imperator.