I hear this a lot. Now the truth..It is absolutely possible to lose weight by counting calories and focusing on your diet alone. But exercise is necessary for body tone, strength and health in so many ways but I am not going into that now.
What I hear is this: I was losing weight and then I started exercising and my weight loss has stalled.
A few reasons. I have clearly explained before that muscle doesn’t weigh more than fat. A pound is a pound. But it takes a lot more fat to = 1 lb than muscle. That is why 2 people who weigh 150lbs can look very different.
But when you work your muscles they need to repair and build and that requires water. So, it is true that your muscles retain water during this time which can stall the scale.
Working out is not a license to eat more or eat crap. Keep that in mind.
Another reason is that people often overestimate the calories they burn in their workouts. This easily leads to the “i just burned x amount of calories so I can have more food”
Any time you make a change in your physical routine you may see a little stall on the scale. It takes your body a little time to adjust. Please keep in mind that you will hit a few plateaus along the way so you need to work through them and not be defeated by them.