Friction

Friction is a pain in the butt when we look at kinematics problems. It is the main non-conservative force, meaning that it changes the total energy in the system. When fining net force or total energy, friction is always a tricky negative sitting in the dark to take points away from innocent physics students. The statement "if friction is negligible" yields an instantaneous sigh of relief, since our job just became that much easier. So why should the universe have friction at all? Well, with no friction, there is no fundamental way, other than air resistance, to reduce a net force. Every object would continually accelerate until it collides with another entity. So unless we all want to accelerate and bump into people forever, friction should probably stay for a while. It would be a pretty nice way to speed date, however.