Why is $\mathsf{P}$ vs. $\mathsf{NP}$ problem considered so important?

Is $\mathsf{P}$ vs. $\mathsf{NP}$ the hardest mathematical problem? 

Why is it so hard?

All I'm looking for is the hindrances to solving $\mathsf{P}$ vs. $\mathsf{NP}$. Are there any theorems that help explain why $\mathsf{P}$ vs. $\mathsf{NP}$ is so hard?