Why does $n^2=\displaystyle\sum_{k=1}^{n}2k-1$ for $n\in\mathbb{N}^+$?

Question

I thought about some random number-stuff recently and I just summed some odd number and realised that

$\displaystyle\sum_{k=1}^{n}2k-1=n^2$ for $n\in\mathbb{N}^+$

I was wondering how you'd prove this and why this holds true.