A student asked what inflection in voice is all about.
If we don't have inflection in our voice, we do not have vocal variation. Our voice sound monotone, which is comparable to listening to a boring professor giving a speech. Usually, downward inflection allows others to perceive that we are making a serious statement, but too much downward inflection can lead to a boring monotone conversation. Obviously, we do not want to bore the person we are talking to, so spice up speech with a little inflection.
Generally, our voice will have an upward inflection at the end of a sentence that is posed as a question. In this case, upward inflection is good; it shows curiosity. But if you use upward inflection with all your thoughts, soon you will sound as if you do not hold much certainty in your own statements. You will constantly appear to be asking for approval, even if that is not how you mean to sound, so be cautious at what points in your speech you make your voice go upward or downward.