Why was losing Rosemary disastrous for the Community?

The previous Receiver's name was Rosemary and her request for release had serious implications on the entire community. The Receiver's primary responsibility is to secure and preserve memories of the past before Sameness. As Jonas discovers, some of the memories of humanity's past are difficult, painful, and traumatic. During Rosemary's training sessions, the Giver found it extremely difficult to transfer physically painful memories to her and instead gave Rosemary memories of loneliness and loss. Rosemary became severely depressed and eventually committed suicide by requesting to be released. Once Rosemary was released, her traumatic, disturbing memories were randomly scattered throughout the community, where the highly-sensitive, naive citizens experienced them for the first time. Rosemary's release resulted in chaos as the citizens struggled to cope with the difficult, painful memories. The Giver tells Jonas,

"It was chaos...They really suffered for a while. Finally it subsided as the memories were assimilated. But it certainly made them aware of how they need a Receiver to contain all that pain. And Knowledge" (Lowry, 104).

Ever since the traumatic experience stemming from Rosemary's tragic situation, it has been written in the Receiver's rules that they are prohibited from requesting release.