Two things influence the amount of friction on an object: the force that is pushing the two materials together and the nature of the surfaces of the two materials. Various interactions between these two aspects can produce variances in friction.
A surface that allows more atoms to come in contact with one another is more favorable for producing higher levels of friction. The inverse is also true, where fewer points of contact between atoms result in less friction. The same relationship holds true for situations where a greater amount of force results in more friction between two objects, while lower amounts of force result in less friction.