Here's why:

* Molarity is defined as the number of moles of solute per liter of solution.

* Concentrating a solution means you are increasing the amount of solute present in the same volume of solution.

* Since you have more moles of solute in the same volume, the ratio of moles to liters (molarity) goes up.

Example:

Imagine you have 1 mole of sugar dissolved in 1 liter of water. This gives you a 1 M solution.

If you concentrate this solution by evaporating some of the water, you'll still have the same 1 mole of sugar, but now it's dissolved in a smaller volume of water (e.g., 0.5 liters). This makes the solution more concentrated, and the molarity increases to 2 M (1 mole / 0.5 liters).