Why do iguanas shake there heads up and down when you get close to them or when another iguana gets close?

Answer

The nodding of its head is typically a warning sign to you or other would be threats. It is a natural reaction when they are excited, agitated or trying to defend themselves or their turf. It is also used when they become sexually excited during mating as well. But I dont think you have anything to worry about there....

Iguanas use head bobbing for different meanings

Rapid head bob or called the shudder is a very bad sign meaning he will fight or guarding his territory. A very slow head bob is a mating bob

There are others that can be a greeting i have heard