The 'negative' on a number line is just a visualization. If you instead, think of the 'negation' as direction (as in a vector), it becomes easier to understand why multiplying two negative numbers makes the answer positive (You've basically turned around twice).
I actually prefer this visualization rather than the one with a number line with numbers going to the left of 0.
To better understand this, and see how complex numbers can be viewed the same way, go through this article, especially up to the first set of slides: https://acko.net/blog/how-to-fold-a-julia-fractal/
I actually prefer this visualization rather than the one with a number line with numbers going to the left of 0.