So, /pony/, it occurred to me, during the regular course of my day, the reason Spike seems to have not developed at all throughout the series.
As you may know, I just got back from Trotcon a few days ago, so I know what the Meaning of Pony is. I was just going about my day, pondering my own insecurities and the meaning of life and pony, as usual. There was an evangelical program on in the background when my mind finally made the connection.
Spike, as a character, represents old, fixed, immovable ways of thinking, and specifically evangelical Christianity.
I should have seen it before.
Let's break this down:
1. Spike is a dragon. Dragons are typically driven by greed and other base desires.
Evangelical Christians are obsessed with overcoming these base desires, so much so that they forget about the true meaning of life - friendship, love. Their minds are overcome by these thoughts, just like a dragon.
2. Spike has no perceptible character development since Season 1.