With exercise as with other things, more is not always better. Contemporary theories on exercise center around the use of Progressive Resistance, where a load is continuously increased in response to increased capacity.
Suppose you want to exercise your biceps to make them larger and stronger. You're doing 2 exercises for biceps. Each exercise consists of 3 sets of 10 repetitions 3 times per week, and you're experiencing some growth. You want more growth, faster growth, so you increase to training your biceps 5 times per week. If more always meant better, you would expect increased growth. Instead, you stop gaining almost immediately, and even start to lose ground. Your biceps become smaller & weaker, even though you are trying very very hard to do the exercises. What's happening?
The answer is, you're over-training.
When you exercise, you actually damage muscle tissue. You disorganize it and stress it. Growth happens as a protective response of the body to this stress, and it happens when you are resting, especially when you are sleeping.
If you train too hard, your body is unable to rebuild, let alone grow, prior to the next training session. Ultimately this means you will lose rather than gain muscle for your efforts.
You should always seek a balance between training and rest which is optimal for your body at any particular time and stage of development. This means the balance will change as your body changes. That's why training routines must continually adapt to be effective. That's why it may be appropriate for you to train a bodypart 3 times per week, or twice, or perhaps just once.
Your trainer, by consulting your carefully maintained exercise log, can help you periodically adapt your exercise plan. If you are a person who likes to train independently, consider scheduling a review session with your trainer once a month, or at least once a quarter, to review your exercises, check for improper form, and tweak that balance between training and rest.