This is something I really didn’t learn until years into my writing, but now I’m keenly aware of it. Because of the way we often talk, we make this mistake in our writing regarding reason and why. The word reason means an explanation. The word why is defined in Merriam-Webster’s as “cause, reason, or purpose.” Maybe you already see where I’m going with this.
If you say, “The reason why I ate that . . . ” you are saying, “The reason reason I ate that.” Now, the word because means . . . you guessed it: why. So now, if you say, “The reason why I ate that is because . . .” you are saying, “The reason reason I ate that is the reason . . .” (or something close to that). Most Word options can be set to detect the reason-why combination and it will flag it, but try to watch out for it. That sentence, by the way, should just be “The reason I ate that sandwich is I was hungry.” Or you could use two sentences: “Why did I eat that sandwich? Because I was hungry.”