Force is a vector, meaning magnitude and direction. Work done by a force is relative to the direction of a force is the scalar value obtained by performing the vector dot product of the force and the displacement (which is also a vector). If something isn't coming out to what you expect when you compute work, make sure you have the right magnitude and direction for everything. The direction of force is not always intuitive.
In the case of a displacement that does not have a uniform force along its length, you would have to use the calculus integral to compute the scalar work done by said force.
The reason work can be negative is because it's possible for something to move counter to one of the forces that is exerted on it (because another force overpowered it or it was already moving in such a way the force didn't get a chance to overpower it). In fact work can be negative, positive or $0$. If you have (2) people playing tug of war the losing team performed negative work while the winning team performed positive work along the axis of the rope. If those people didn't change their elevation relative to the center of earth's mass, the gravity exerted by the center of mass for performed $0$ work while they were playing, even if they temporarily were dug into the ground (having a net displacement of $0$ results in a net work of $0$ - even if you temporarily had some displacement).
You could probably look at work as "when push came to shove how effective was the force at displacing"?
Assuming a stationary medium relative to an external observer, if you're sliding (relative to the stationary medium/observer) along some +x axis then any friction you experience would be accelerating you in the opposite direction, then the force friction delivers would be along the -x axis. Friction would only act on something moving to accelerate it in the opposite direction its current velocity (which is a vector, because velocity is speed and direction), so friction, from its perspective, always has a negative displacement it works over, so the work friction does is always negative.
If something is already moving really fast it's also possible that all forces perform negative work (at in a simplified model where you have a direction such that no force acts positively in it). Force has a say in the change in momentum once it starts acting on something, but doesn't have any say over what the momentum was prior to it acting on it, and for that reason the work done by that force can be $0$, negative or positive (as oppose to the net change in momentum which is in the same direction as the net force). Of course if you started with $0$ momentum, then the work done by a positive net force cannot be negative, and if there was any change in momentum the work by that net force will be positive.