Tiresome as they are, these “why” questions are actually very sweet and valid. The child is genuinely seeking information or even wisdom at this age. But as children get older and more capable of verbal combat, a “why” question is almost never a kind-hearted way to start a conversation, especially with a sibling.
One sister to another: “Why are you wearing that outfit?” Ouch. Or, "Why did you put that comb on the bathroom counter when I am trying to brush my teeth?"
You see my point. Those questions were a big ol’ dagger to the heart dressed up in disguise as an innocent pondering. The sister responds to the question as a hostile witness and then an argument has broken out. Children, please. I have announced to my people that I want them to be aware of letting out the flood gates that lead to an argument by rarely starting a sentence with the word "why."
The Bible says to “let your speech always be gracious.” I’m not asking my children to be perfect. I’m not asking them to adore every decision, opinion, or annoying habit from their siblings. I am asking them to “take captive every thought to the obedience of Christ.” I have told my children to make an effort to:
1. Notice when they start to feel aggravated at a sibling.
2. Try not to start a sentence with “why” right then.
3. Wait until they are no longer annoyed and then say, with kindness, and possibly a hug and a smile: "I would really appreciate it if you would--." Fill in the blank.
Jesus asked a lot of questions. He wasn't seeking information. He in His perfection was rightfully challenging people on their sin and asking them to reflect on the condition of their hearts. "Why are ye fearful, O ye of little faith?" He said in Matthew 8:26. Eventually, children will mature into being able to ask important "why" questions that are thought-provoking and necessary to our times, such as, "Why has America sent American military personnel to another continent to expose them to the Ebola virus?" or "Why is our president cozy with Iran?" Those are good questions.
Until then, not dishing out criticism disguised as asking “why” questions is important in personal relationships because it honors God. Having lots of practice in controlling themselves in this way with siblings matters because someday, hey. It’s going to be their husband putting that hairbrush in the wrong spot on the bathroom counter.