Why will The Giver not go with Jonas ?

When Jonas decides to leave, he pleads for The Giver to
come with him.  The Giver responds that he is old and not strong enough for the
journey.  He also says that the community will need
him.

After Jonas leaves, they know he will face harsh
conditions.  Long hours of running, harsh weather and lack of food would be very hard on
The Giver.  He would likely slow them down.  Gabe is enough of a burden, since he too
needs to eat and be kept warm and cannot fend for himself.  The Giver has also given
Jonas many of his memories, possibly leaving him not enough to sustain himself and
them.

The main reason The Giver cannot leave is that when
Jonas leaves, all of the memories he has will be released back to the community. Chaos
will ensue unless someone is there to help them.  With both Jonas and The Giver gone,
all of the memories of pain and suffering would go back to the people.  They would be
confused and not know what was happening to them, and without The Giver there would be
no one to help them through it.