Why do we get scars?
Scars are fibrous tissue that is left after an injury. The body does its best to fix a body part after an injury and scars are their best attempt at doing so. It would be pleasant to think we could have Wolverine type of healing capacities but we don't and the best we can do are those scars.

The ability to completely regenerate the damaged tissue depends upon the specific tissue. Also, some species are better of than others at regenerating their tissues. Such differences relate to the concept of cellular specialization.

For more information you should have a look at the wikipedia articles


Wikipedia > scars
Wikipedia > Regeneration
Wikipedia > cellular specialization


and the Biology.SE posts


Can a lizard shed a regrown tail again?
Can the regeneration mechanism of lizards be used in humans?