Here's why:

* DNA as a blueprint: DNA contains the genetic instructions for building and maintaining an organism. The more similar the DNA sequences, the more similar the instructions, and therefore the more similar the organisms are likely to be.

* Evolutionary relationships: DNA changes over time due to mutations. The accumulation of these changes provides a molecular clock, allowing scientists to estimate how long ago two species shared a common ancestor. The more similar the DNA, the more recently they shared a common ancestor, meaning they are more closely related.

However, there are some important caveats:

* Convergent evolution: Sometimes, unrelated species can evolve similar traits due to adapting to similar environments. This can lead to similar functions, even with different DNA sequences.

* Different genes evolve at different rates: Some genes change very slowly, while others accumulate mutations quickly. Therefore, comparing specific genes might not always accurately reflect the overall evolutionary relationship between species.

* Non-coding DNA: A large portion of our DNA doesn't code for proteins and has less direct influence on our traits. It can still be useful for evolutionary analysis, but needs to be interpreted carefully.

In conclusion: While DNA similarity is a powerful tool for understanding evolutionary relationships, it's important to consider these nuances when interpreting results.