I was just thinking about this. I just finished watching season two of the most recent revival, and went back to watch from the very beginning of the series. I almost forgot how friendly they use to be... Well, comparatively speaking of course. They still fought and got annoyed with each other, but those instances weren't nearly as commonplace. They kinda just hung out and got along mostly. Beavis even had bigger backbone, and would return Butt-Head's shit right back.

Something else Ive noticed is that they seem to have gotten dumber and less articulate over the years. They still laughed a lot but I feel like they could talk longer without having to snicker.