Why is kevlar the strongest substance/element in nature? Explain.
Kevlar is a brand name for a para-aramid synthetic fiber (usually shortened to "aramid fiber"). It was invented by DuPont and has been around since the early 1970s (see the first link). It is not an element, it is not found in nature and it isn't the strongest substance.
Kevlar is an amazing fiber with many uses, including many industrial and safety applications. It has very high tensile strength and a very wide temperature range. Its tensile strength is mainly because of the bonds that form between molecules. It has very low heat conductivity and is used for many high strength insulation applications and applications requiring high strength materials capable of withstanding extreme temperatures (especially low temperatures since Kevlar's strength degrades at high temperatures).
For how impressive Kevlar is, there are stronger substances available now and more under development. Carbon Nanotubes (see the second link) have been available for about 5 years now, and have tensile strength that greatly surpasses Kevlar.
As for commercially available products right now, Zylon (generically known as PBO) is considered the strongest material presently available (see the third link).
I hope this answers your question.