Why do solar panels need to be the same voltages when in parallel?
A solar panel is, fundamentally, a photodiode*. If you forward-bias that diode, current will flow through it.

To elaborate a bit: You may have used photodiodes for circuitry that has to respond to light. When used for light sensors, they're generally reverse-biased and operated in the photoconductive mode, but they can also be used in the photovoltaic mode, where they are allowed to develop a voltage as a consequence of light striking them. If you scale this up to a point where it can deliver significant current, what you get is a solar panel.

*yes, there are differences in how they're made and what they're optimized for, but they are basically the same idea.