Why should you generally use active voice rather than passive in your writing?

because active voice is more direct than passive voice and is easier understood. Given the short memory span of the listener / reader it is easier to see the whole picture when active voice is used. Likewise, active sentences are more cohesive.

For example: Mom is cooking pasta (main thought: mom is cooking, natural question is what? which is immediately answered by the Word pasta

as against

Pasta is being cooked by mom. (the main thought becomes Pasta is being cooked and the natural question of course will be so what? and not by whom the phrase by mom then somehow loses its relevance)