Why are so many worried about how the troops feel about dissent in the U.S.?

Because the very blood pumping thru their bodies are being put at risk and the only thing, and a rather small thing at that they would like to hear the words thank-you. Thank-you for being so brave as to volunteer for this. But hearing the protests no matter how nicely done is telling them that they don't have these people standing behind them because they don't believe in what they are doing. Very demoralizing.