Yes, we have a theoretical understanding of why time dilation occurs based on the principles of Einstein's theory of relativity. Time dilation is a consequence of the fundamental postulates of this theory:
The principle of relativity: The laws of physics are the same in all inertial reference frames. An inertial reference frame is a frame of reference in which an object is either at rest or moving with a constant velocity.
The constancy of the speed of light: The speed of light in a vacuum is the same for all observers, regardless of their relative motion.
From these postulates, it follows that the measurement of time depends on the relative motion of the observer and the object being observed. When two observers are in relative motion, their measurements of time will differ.
To explain time dilation, we consider two observers: one who is stationary (let's call them observer A) and one who is moving at a high velocity relative to observer A (let's call them observer B).
Observer A perceives time passing uniformly and normally. However, observer B, who is moving at high speed, perceives time passing more slowly compared to observer A. This difference in the perception of time between the two observers is what we refer to as time dilation.
The reason for this phenomenon lies in the interplay between space and time. According to relativity, spacetime forms a four-dimensional fabric where the geometry is affected by the presence of mass and energy. When an object moves at high speeds, it interacts with this fabric, causing a distortion in the relative flow of time.
To summarize, time dilation occurs because the speed of light is constant for all observers, regardless of their relative motion. As an object approaches the speed of light, its motion through spacetime causes a stretching or slowing down of time as observed by a stationary observer. This effect is a fundamental consequence of the theory of relativity and has been extensively confirmed by experimental observations.