My four year old grandson was watching a cartoon whose content was decidedly teenage, dating and so forth. At one point he laughed and I asked him if he thought that was funny. “Yes,” he said, and then added thoughtfully, “… and I don’t know why.”
He used “and I don’t know why” with exactly the perplexed but resigned tone an adult might use it to punctuate a mystery like dating the wrong people or eating cake for breakfast.