Coming in contact with a Mes can cause a certain hopelessness and lack of Simchas HaChaim. Indeed, an Avel has to eat a Seudas Havra’a. Perhaps: מַיִם חַיִים אֶל כֶלִי (Numbers 19:17) “live” waters [from a spring] can symbolize being alive and bubbly; “therapy” for the sadness from encountering a Mes. The Parah Aduma is called Chatas, because “sadness” is indeed in the category of a “sin” and worse.
Actually, the word “Chet” means “missing the boat”, as it says about the archers from Shevet Binyamin who were such expert sharp-shooters: וְֹּלא יַחֲטִא they never “missed” their target. Lacking in Simcha and Chi'yus is a kind of Pikuach Nefesh, which is: דוֹחֶה כָּל הַּתוָֹּרה of utmost urgency. The person missing the basics of Simchas HaChaim has a different Shulchan Aruch.