No one knows, but the facts make you wonder...
1. Everything in the universe is moving at the speed of light. You, me and the rest of the mass are moving at the speed of light along the 'time' axis. Rays of light move at speed of light along the 'space' axes.
2. Every time you speed up along the 'space' axes, you slow down along the 'time' axis, and vise versa. E.q. your total speed in space-time cannot exceed the speed of light.
3. Time doesn't 'flow' for rays of light, because they are moving at the speed of light along the 'space' axes. If you were a photon you wouldnt have the time to realize that you're alive. Actually, there would be exactly 0 time between the moment you are born and the moment you die. During that 0 time you could travel across the entire universe.
4. Some suggest that this is exactly whats happening when you fall into a black hole (past the event horizon). 1 dimension becomes unavailable for traveling, instead you're dragged along it at the speed of light. When you move aside, you 'fall' slower. Given that, time could 'flow' because of gravity as well.