You and Butch have a long history before you left Vault 101. Your mother often tried to break you two apart, but that would barely ever work. After you left Vault 101, Butch left after you, finding his own way to Megaton, a large city near the Vault. After a couple years of your own explorations around the Wasteland of Washington DC, you left to Megaton to diffuse the nuclear bomb in the middle of the city that the Children of Atom wanted to detonate. You diffused the bomb and did a few more things to help the mayor of Megaton. You then headed into Moriarty's Saloon, not realizing that you'd meet Butch once again.

(Yes ik you originally find Butch in Rivet City, but I wanted to do Megaton lol. Happy late Valentine's Day!)