Why Is Base Resistance Reflected in Parallel With RE When Finding BJT Rin From Emitter?

Hi guys, I've read about BJT and its internal resistors, but I'm a lil confused about when I look from the emitter, why the bjt reflected resistors of the base are in parallel on what we find on emitter and not in series? I mean the reflected resistors of the base are in parallel with RE and not in series why? I've attached an example of what I'm trying to find.I'm trying to find the Rin looking from the emitter of the bjt .. any help with good explanation?!

Think about what happens if you inject a small current into the output. Some of that current goes into 100-ohm Re, and the rest of that current goes into the transistor's emitter and hence into the reflected resistors of the base.