Anglos who come to America are struck by the number of people who revel in describing their achievements. It deserves explanation. Here are some reasons why folk might insist on telling you all about the great things they've done.
1. It's the way they've been brought up. There's certainly a cultural component to talking about oneself. The English are taught to take a perverse pride in being excruciatingly humble. the same seems to be true in Japan and the Mid-west. I haven't discerned any regional patterns of American self-aggrandizement, though.
2. They're selling themselves to you. If people feel that they can prove themselves by describing their achievements, they'll do it. If it leads to increased respect, it's in their best interests.
3. They're selling themselves to themselves. This has always been my favorite explanation: people who revel in themselves have a low sense of self-worth. I'm not so sure any more. The very prevalence of the truism that blames all sorts of social ills on people's low self-esteem has made me increasingly skeptical of it. There's even evidence on the other side, now. It's often said that bullies have low self-esteem. However, New Scientist reports that bullying others has social value (Clare Wilson, 5 March 2004, "Teenagers special: Bully boys"). Anthony Pellgrini found in a study of 138 school children that bullying raised a perpetrator's social status (Journal of Experimental Child Psychology, vol 85, p 257). These kids know what they're doing. According to Pellegrini in this story: "Boys have high status with their male peers if they're bullies, and girls like them"
4. They're just bubbling over with enthusiasm for their work, and they love telling you about it. People who are goal oriented achievers will frame their work in terms of accomplishments, and a litany of triumphs will pour out without any ulterior motive.
I'm beginning to think that enthusiasm (#4) accounts for quite a lot of pride expression, particularly in America. Overt enthusiasm is encouraged here, and so enthusiasts are more visible. Tie that to a culture of selling (reason #2), and one can begin to explain #1.