I don't know how but the rubber grip on the xbox controller joysticks wears off. Especially, the left joystick.

Fortunately, Sugru can fix that.

Step 1: Apply Sugru

First make sure the joystick is clean. Remove any residue rubber.

Open a (mini) pack of sugru and roll it into a sphere.

Flatten it and place on joystick.

Smooth-en out the edges and the surface.

Step 2: Finish

The final result should look the the attached photo.

Make sure the Sugru dries before you use your controller.