Canonical Reasoning: He is able to defy gravity when on a planet orbited by a yellow sun.
Scientific Reasoning: Superman's species, as illustrated by the mythology, was assumed to have been highly evolved by comparison to the standards of human primates, utilizing their crystals they have tapped into the use of highly dense quantum computer's that allow them access to their entire genome, and absolute control over the reproduction of any member of their species. These same kryptonian crystals, now genetically embedded into their tissue growth cycle, are the same modern scientists have recently discovered can help a basic electronic generator harness ZERO POINT energy. However, scientists have also discovered that it takes a certain quantity and type of these crystals to manage the power needed to produce ZERO POINT energy, thus explaining Superman's telekinetic constraints. Kryptonian crystals flow through the veins, and more specifically the DNA of a Kryptonian, and are powered by the photocells in their skin that gather radiation from the sun. This Kryptonian blood theory can be illustrated by the Smallville episode "Conspiracy", in which Clark Kent mistakenly transfers his own powers to Zod through a minor blood transfusion. Because Superman is living beneath a yellow sun obviously his molecules are denser than if he were living beneath a red sun, however the weight of any object in an environment can be altered by ZERO POINT energy in order to produce anti-gravity, but in the case of Superman, he was raised entirely on Earth thus training his techno-telekineses to only have an effect on his own body. This also inadvertently explains how even though the crystals on Krypton are filled with nano-computers to help generate their advanced technology and power their ZERO POINT energy, any crystals of another extreme of quantum radiation could potentially disrupt the crystals either inside or outside of a Kryptonian's body, IE Green, Blue, Black, Red, or any other color of Kryptonite. So, to simplify it into layman's terms, Superman can fly because he has mental and bodily control over a limited amount of ZERO POINT energy.