my amateur psychoanalysis, given my recent experiences:
Alex is the sparkplug of this board. Others here get it started but Alex keeps it moving. Alex likes to be the lightning rod, the Attention Whore, if you will. His deliberately provocative threads such as "so I was jerking off and ..." bear this out in spades. In the immortal words of Oscar Wilde, as interpreted through Monty Python, "the only thing worse than being talked about is NOT being talked about."
We rag on Alex because we know he can take it, and dish it back many times over. In essence, we rag on Alex because we like him.