why do the states of matter differ ?

The different states of matter are due to the variation in the characteristics of their constituent particles. The three states of matter, solid, liquid and gas differ in the way their atoms are arranged, their intermolecular distance and the intermolecular force of attraction between their particles.
Solids:  The particles (atoms or ions) in a solid are held by strong intermolecular forces of attraction.  The particles do not easily move away from each other and are present at specific points in the substance. Hence, they have a definite shape, distinct boundaries and fixed volumes.  As the molecules are present close to each other they have negligible compressibility. They are rigid and therefore will break rather than change their shape if force is applied on them.
Gases: The intermolecular attractive forces are weak and particles are loosely held.  They occupy all the space available to them and thus do not have a definite shape and volume. They take up the shape of the container they occupy.
Gas particles move around randomly at high speed i.e. exhibit Brownian motion. Due to this movement they hit each other as well as the walls of the container they are placed in and these collisions generates a pressure called the gas or vapour pressure.
As they have large intermolecular spaces between their particles, gases can easily occupy compact spaces and hence are compressible.

Liquids particles exhibit characteristics in between a solid and a gas. Particles of a liquid can move freely and intermolecular space between liquid particles and intermolecular forces of attraction between the particles is intermediate between that of solids and gases. Hence, liquids have a fixed volume but not a fixed shape and take up the shape of the container in which they are kept.  They are not rigid as they can change their shape and are able to flow.

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