Why are people so loud ?

I don't understand I work in a large organisation open office kind of floor plan I just don't get why people speak soo loudly at work It's really weird? I've worked at other offices and everyone is quiet and respectful but at this office everyone is so loud I don't get it ?? Why don't people try and be quiet and respectful ? Today I was trying to concentrate and3 colleagues behind me were speaking so loudly and laughing at jokes