Answer:
When Mr. Frank says, "We don't need the Nazis to destroy us We are destroying ourselves", he means they are destroying their relationships by arguing. They should not let food get between them because all they have is each other to lean on now. I agree with this because they do not know how long they will be there together. They need to get along. Somebody might hear them arguing and they do not want to get caught. They have to be quiet during the day so arguing would only make that more difficult. I'm sure going into hiding was difficult emotionally already and add having to worry about other people stealing food. They should divide everything evenly and agree not to go behind each other's backs. Everyone needs to agree on what happens while they are in hiding together. If they got caught by something they did they would never forgive each other.
Hope this helps!