Guy's Behavior

What does it mean when a guy defends you from one of his close friends?

So I was with this guy who's a pretty good friend of mine and one of his close friends. The three of us were talking and messing around, and I started teasing his friend over something he said. but at the same time his friend turned and called my friend a bitch for stealing his drink. Except my friend thought he had called ME a bitch and he got really serious with his friend and told him to not speak about me like that. I thought it was sweet that he was defending me against his friend because the two are practically like brothers, but it was also kinda weird because I'd never seen him so serious and he's always constantly teasing me so I didn't know why he'd be so upset about what his friend said (even though it wasn't directed at me). So was he just being nice, or does it mean anything more?
What does it mean when a guy defends you from one of his close friends?
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