From my studies of Einsteins work I am under the impression that the most important motive for his works stems from his interest in understanding physics from the point of view that matter exists of atoms and molecules that follow paths in a space-time continuum.
In the sense that he was strongly influenced by the idea of a deterministic-materialistic universe - the Kant-Laplace universe. This point of view easily explains why he couldn't accept quantum physics where the concept of probablities - without underlying mechanism - supersedes determinism.
I believe the comment of user346 is important: "[...] the atomic hypothesis was not universally accepted before 1905 [...] a direct link between the atomic hypothesis and the observed macroscopic properties of matter".
In my view his work on the SRT has the same motive: understanding space-time relations from the point of view of light rays imagined as moving more or less like macroscopic objects.