This question has a cascade of answer, that is the primary answer induces a secondary question that in turns induces other questions.
The primary force that motivates seafloor drift is rising magma at the ocean rifts. As the magma rises it flows off to the side (volcanos can only get so high before their vertical weight stops further rising and forces basal magma off to the side.)
Why does magma rise?
Magma rises because it is hot and fluid. Hot ANYTHING is less dense than cold ANYTHING and will float up.
Why is magma hot?
The material at the earth's core contains unstable neucleotides (unstable neucleotides are everywhere) and when they decay they release heat. Being at the Earths core this heat has no where to go. The mass of the earth above it is like a huge insulating blanket so the centre of the earth keeps its heat and gets hotter and hotter until the rock at the core turns into magma and floats up.