The point of training is to better your body. However, more and more is not the answer in the long term. Overtraining refers to a combination of stresses such as too much training load, a heavy load at work, personal stresses and emotional burnout. Some signs of overtraining include weight loss with a lessened appetite, loss of motivation in physical activity, an increase in muscle soreness and exhaustion and increased anger and emotional outbursts, to name a few.
It would seem logical to keep piling on more and more physical effort to keep improving your body. But the fact is that humans are biological machines, and therefore need rest and lower stresses. Here is where periodization comes in.
Periodization is the planning of a workout program that starts in phase 1 to target the endurance fibres in the muscles, with lots of reps and a low load. Gradually over several weeks, the load is increased, but the volume (sets x reps) is decreased. In the transitional phases, the program switches from targeting the endurance fibres to the strength fibres. Rest and recovery between sessions are absolutely essential.
A trainer should be planning this transitional programming, with about two weeks in between changes in load or volume. It takes that long to adapt and get prepared for changes If you are having problems with excessive muscle soreness that persists longer than a day or two, mention it to the trainer and some regression is in order. When forward changes do happen, the trainer should limit the stress to about 5% increase.