Your question is interesting and it needs some explanation. Here it is....

Regeneration is the process by which some organisms replace lost body parts and a number of regenerative mechanisms have been evolved by different species.Regeneration is most common in invertebrates, occurring in almost all coelenterates and planarians, most annelids (segmented worms), and many insects.

Many starfish can drop off arms as a defensive reaction. They can then regenerate new arms to replace the old ones. If a starfish is cut in two, each of the pieces may regenerate into a new animal.

Among vertebrates, no mammals have the ability to re-grow lost limbs or tails, but some species can regenerate other peripheral appendages, (e.g. a deer's antlers) or internal organs (e.g. the human liver).

In most fishes and salamanders, limited regeneration of limbs occurs, and tail regeneration takes place in larval frogs and toads (but not adults). The whole limb of a Salamander or a Triton will grow again and again after amputation. In reptiles, Chelonians, crocodiles and snakes are unable to regenerate lost parts. But many (not all) kinds of lizards, geckos and Iguanas possess regeneration capacity in a high degree. Usually, it involves dropping a section of their tail and regenerating it as part of a defense mechanism. While escaping a predator, if the predator catches the tail, it will disconnect. The tail lays flopping in the predator's mouth or on the ground. While the predator is occupied or distracted by the wriggling tail, the reptile runs away. The skin, muscles, blood supply, nerves and bone separate at almost any place along the length of the tail (below the reproductive organs). Later, when growing a new tail, it will not include all of the tissues and structures of the original one. Instead of the segmented vertebrae, a long tapering cartilaginous tube develops within which the spinal cord is located and outside of which are segmented muscles. The spinal cord is replaced by an epithelial tube, which gives off no nerves.

Very frequently super-regeneration occurs, the amputated limb or tail being replaced by double or multiple new structures.

While the loss of the tail may be natural, and may save a lizard's life, it isn't without cost. It is stressful to the lizard, especially if that lizard stores critical fat deposits in the tail, such as leopard geckos. Should a lizard be attacked twice, it is advantageous to have a re-grown appendage. However, tail regeneration is energetically expensive and can also result in lowered social status. Still, it is better than being someones dinner.

The less stress the lizard has to deal with, the faster the stump will heal and, if the tail is going to regenerate (they do not always do so), it will do so fairly rapidly.

Most lizards can regenerate more than once. The glass lizard is only able to shed and regenerate the tail once in its lifetime. When the new tail grows back, it will be smaller than the original. Unlike other lizards the monitor's tail does not break off and regenerate.

Regenerating tissues apparently follow a strict polarity, growing back in the proper orientation to the rest of the body. Since the most commonly lost structures are limbs and tails, the pattern of growth is usually outward from the body, suggesting that tissues more proximal than the injury contain all the necessary information to replace the lost part, but not those closer to the main trunk of the body. In some cases, however, as in fish fins, regeneration may occur in both directions. Regeneration is part of developmental biology and involves many unresolved problems. Scientists are trying hard to understand the molecular basis of regeneration.