The main source of attraction between two metals, or adhesion, is what causes friction, and any increase of friction requires an increase in adhesion. Friction itself is the force of resistance against two objects as one object slides or rolls over another. This force increases the amount of energy required to move an object a set distance.
Increases in friction can be caused by a change in the surface of the objects in question or by increasing the pressure or volume of the gases or liquids surrounding it. Atmospheric conditions can increase friction by a small amount. An object with higher mass is affected by gravity to a greater degree, increasing its load and the amount of friction it puts into anything it moves against. Objects of sufficient size break up in the Earth's atmosphere because the friction creates heat and resistance that destroys the object.
Friction as a force is not always a factor in the motion of objects. In the vacuum of space, an object may be in motion for long periods without experiencing friction, allowing the object to move at a steady rate in a single direction until it meets an obstacle. Friction is only introduced when this object meets the mass of another object.