Imagine being in a trial. The defense attorney is arguing a wonderful case for his client, but stops to listen to a noise behind him. The attorney turns around and sees the defendant talking about the stock market and sport scores with a friend sitting next to him. How does this scene look to you? What do you think the judge will say? Our great Sages say this is what it looks like when someone talks during the Repetition of the Shemone Esray. The Sheliach Tzibbur (prayer leader) is praying for the benefit of the congregants. If one of those congregants talks during this time, it looks like he does not even care that someone is trying to save his life. This is certainly not something we want.