There are studies that show in order to read and understand another's thoughts promptly, its first most important to recognize the fisrt and last letters of each word, as long as most of the rest letters are included and the word has the correct number of letters, regardless of if they are in perfect order or not, the brain understands/recognizes the word and moves on to the next word. CAPITALS upset that process. It's not the stated perceived "yelling", it's the awkwardness of reading all capitals that is the issue. Your grade school teacher and spelling bee winners are likely to completely disagree with this thinking. Mark Twain also had something related to say on this topic.
An Exhibit below from the San Fransisco Exploratorium:
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