Why would senior fitness instructor push us to do lunges with weights like a drill sergeant?

For Strength: Hard to tell if you're upset with the effort involved (weighted lunges) or the instructor's in-your-face attitude. Adding weights to lunges is a great option, if somewhat advanced, to strengthen the thighs and improve core strength. The option to do them or not should be up to you and based on your fitness level and comfort. Some people like being "pushed" to excel, many others don't..Up to you!