Here's why:

* Lines of force represent magnetic field strength: The density of magnetic field lines (how close together they are) indicates the strength of the magnetic field. More lines in a given area mean a stronger field.

* Strong magnets have a denser field: Strong magnets have a denser magnetic field, meaning they have more lines of force packed into a smaller area.

* Visualizing the lines of force: Imagine a bar magnet. The lines of force leave the magnet at the north pole and enter at the south pole. A stronger magnet will have more lines leaving the north pole and entering the south pole.

In summary: A magnet with many lines of force is a strong magnet because its magnetic field is concentrated and dense.