What happened when Jonas soothed Gabriel during the night, and why did this frighten him?

Towards the end of chapter 14, Gabriel begins to cry in the middle of the night and wakes Jonas up. Jonas then takes Gabriel from his crib and rhythmically pats the infant on the back to calm him down. As Jonas is rubbing Gabriel's back, he begins to remember a breezy day and imagines a sailboat gliding on a clear turquoise lake. As Jonas is thinking about the peaceful scene on the lake, the image in his mind starts to become dimmer and dimmer. Jonas then suddenly realizes that he is transferring the memory of the sailboat on the lake to Gabriel, who is now sleeping peacefully, and instinctively pulls his hand away. Jonas is initially shocked by the fact that he can successfully transfer memories and is astonished to discover that Gabriel can receive the memories. This incident briefly disturbs Jonas, who can barely remember the peaceful day on the lake anymore because he transferred most of the memory to Gabriel. Later that night, Gabriel begins to cry again and Jonas soothes Gabriel by transferring the rest of the memory to him.

Jonas volunteers to keep Gabriel in his room one night, so his mother and father can rest. Although Gabriel continues to grow and make progress, he is restless and whimpers at night. When Jonas hears Gabriel begin to move around in his crib, he places his hand on Gabriel's back to pat and soothe the restless baby. As he comforts the baby, something unusual happens. Jonas begins to think about a memory given to him by the Giver. Jonas notices that the memory seems to have a calming effect on the baby, but Jonas is losing the memory as it passes to Gabriel. Jonas goes to Gabriel a second time when Gabriel becomes restless. This time, Jonas intentionally places his hand on Gabriel's back and releases what is left of the memory to Gabriel. As a result, Gabriel sleeps.
Jonas is frightened because he has just discovered he has the power to pass his memory to Gabriel. He is worried because he is a Receiver, not a Giver. In addition, Gabriel has not been selected to be a Receiver.