Here's why:

* Friction is a force that opposes motion. It acts in the opposite direction of the object's movement.

* Friction converts kinetic energy (energy of motion) into heat energy. This means the object loses energy and slows down.

Example: Imagine pushing a box across a rough floor. The friction between the box and the floor creates resistance, making it harder to move the box and causing it to slow down eventually.

However, there are situations where friction can seem to speed up objects:

* Static friction: Before an object starts moving, static friction prevents it from moving at all. Once enough force is applied to overcome static friction, the object begins to move. In this case, static friction might seem to "speed up" the object, but it's actually preventing it from moving until the applied force is strong enough.

* Rolling friction: This type of friction is much lower than sliding friction, which can allow objects like wheels to move more efficiently and potentially speed up, but it's still a force opposing motion.

In general, friction is a force that slows down objects, but it can have complex interactions that make it seem like it's speeding them up in certain situations.