Why do we sometimes end up screaming?

[aparlicle]

I am curious to know why sometimes we as people can get very loud during conversation and end up screaming. Sometimes I myself find I am engaged in a conversation at normal volume and by the end of the conversation I am really screaming. But everyone else is screaming too as the conversation continues. While in the conversation it is not bothersome but when I hear other people screaming it drives me crazy. I hate loud people.
How can I stop myself and those around me from screaming?