Because humans generally need some sense of belonging. It's in our biological firmware because groups tend to survive longer than individuals.
Cameron: Why did you hire me?
House: Does it matter?
Cameron: Kinda hard to work for a guy who doesn’t respect you.
House: Why?
Cameron: Is that rhetorical?
House: No, it just seems that way because you can’t think of an answer. Does it make a difference why I think I’m a jerk? The only thing that matters is what you think. Can you do the job?
Cameron: You hired a black guy because he had a juvenile record.
House: No, it wasn’t a racial thing, I didn’t see a black guy. I just saw a doctor…with a juvenile record. I hired Chase ‘cause his dad made a phone call. I hired you because you are extremely pretty.
Cameron: You hired me to get into my pants?!
House: I can’t believe that that would shock you. It’s also not what I said. No, I hired you because you look good; it’s like having a nice piece of art in the lobby.
Cameron: I was in the top of my class.
House: But not THE top.
Cameron: I did an internship at the Mayo Clinic.
House: Yes, you were a very good applicant.
Cameron: But not the best?
House: Would that upset you, really, to think that you were hired because of some genetic gift of beauty not some genetic gift of intelligence?
Cameron: I worked very hard to get where I am.
House: But you didn’t have to. People choose the paths that gain them the greatest rewards for the least amount of effort. That’s a law of nature, and you defied it. That’s why I hired you. You could have married rich, could have been a model, you could have just shown up and people would have given you stuff. Lots of stuff, but you didn’t, you worked your stunning little ass off.
Cameron: Am I supposed to be flattered?
Furthermore, no accomplishments can be entirely separated from genes. Your achievements wouldn't have been possible if you had the genes of a bacterium rather than human genes.
Accomplishments also tend to depended on a mixture of luck and a good starting environment (like not being born into poverty).
Pride is the opposite of humility; and of the two it is humility that is more reasoned.
If you've lived your entire life with people complimenting you on your looks, height, eyes, complexion and rewarding you for them, it's hard not to feel like you've accomplished something.
If you're smart or tall, for example, that's deeply tied to how people perceive you and your identity.
In some cases, like intelligence, it's easier to justify the pride by saying you worked hard to get it -- even if it's more a function of genes, upbringing, etc.
In other cases, people are happy to have a good quality, and perhaps the unearned "pride" is a flaw that we are all susceptible to.
The other half is passing them on. Having "good" genes makes it more likely to be considered an attractive mate to a larger percentage of potential mates. And just being seen as an attractive mate can positively effect one's status as well as the implicit biases of others.
People seem to derive utility from membership in a social group, or conforming to this group prescriptions ("thou shalt not like Australian coffee???") so I believe this is just an extension at the gene level.
Phrase “im proud of
The main thesis of the book is that we are very often not aware of our real reasons for most of our behaviors. Our behaviors are optimised for living in a social group and very often, from the point of view of natural selection, it is useful if we are not consciously aware of our real motivations.
The book is split into two sections. The first, entitled 'Why We Hide Our Motives' includes an introduction to the subjects of animal behaviour, signalling, social norms and self-deception. In the second section, title 'Hidden Motives in Everyday Life' each chapter covers an aspect of human behaviour and describes how it can be explained through the framework of signalling and self-deception outlined in the first section. The chapters in this section cover body language, laughter, conversation, consumption, art, charity, education, medicine, religion and politics.
99% of what people do or say is bullshit it's all competition
Why is being proud of anything now acceptable in our society? That’s the better question. Pride will find reasons to justify itself, if you allow it. People are proud of all kinds of outlandish things.
People are proud because it is who they are, the media, peers, society in general can twist and turn but in the end, you didn't pick them but your genes became you and are you.
Random chance and mutations are completely ignored of course.