I know this thread is old and dead but you did want a simple intuitive answer to your inquiry…and so here it is. Imagine applying a source signal directly at the emitter. Initially the source would just see the base impedance (plus emitter resistance re) in parallel with RE…but then all of a sudden…the transistor wants to yank on the source current (because of the current gain) – it wants to draw way more current from the source than it had originally bargained for (reflecting a drop in impedance as seen by the source). Or at least that's how I see it.