why does email group all mail from one person together and send the entire group

Instead of having each email separate all email to one person are now grouped together and the entire group is sent each time a new email is sent. For example I send Sam an email and even if I don't reply from the original email if I send him another email it will be grouped with the first email and resent throughout all my emails to Sam unless I delete the first one. How can I prevent this from happening. sometimes I don't want all conversations sent should I want to forward an email from Sam to Sue all my emails from Sam will got to Sue and I may not want sue to see some of my email to Sam.