How can being proud of oneself sometimes have negative effects?

I'd say being overly proud can backfire when it makes people stop improving. That's largely because they start thinking they're already good enough, so they don't push themselves further. Take my experience learning video editing last year - at first I felt quite proud when I made my first decent tutorial video. But that pride actually slowed me down for a while because I stopped actively seeking feedback or learning new techniques. It was only when I saw others' more professional work that I realized I still had room to grow. That's the tricky thing with pride - it can quietly take away your motivation to develop if you're not careful.