When learning measurement basis, my teacher told us $|0\rangle=\frac{1}{\sqrt{2}}(|+\rangle+|-\rangle)$ and said that we can derive it ourselves. Along this, he also mentioned $|+\rangle=\frac{1}{\sqrt{2}}(|0\rangle+|1\rangle)$.
I understand that when we visualize those vectors on a bloch sphere, $|0\rangle$ lies in between $|+\rangle$ and $|-\rangle$, and if we normalize the coefficient, we would get $\frac{1}{\sqrt{2}}$. However, I'm confused how we know that the phase is + ($|+\rangle+|-\rangle$) instead of -? Is this just a definition for $|0\rangle$ or is it backed by a deeper reason?