It can have something to do with many things - for one, Aries is ruled by Mars and Taurus is ruled by Venus. When other planets in these two people's birth charts are compatible, it is only natural that the attraction between Mars and Venus is very magnetic, regardless of sun-sign compatibility. I am a Taurus in a relationship with Aries, and I personally feel this plays a big part. I can be quite independent, stubborn, refusing to be dominated... but my Aries just brings out the Venus in me, softens me. He very naturally compliments me that way, as I do for him, too; I was always "hard to tame" and he was always very shy - but we balance those qualities in each other. I really think that Mars-Venus ruled signs can be crazy attracted to each other like to no-one else, if the signs in their birth charts are right and all else is in order, too.
I have to be honest; the theory of Aries bringing excitement to Taurus sounds very plausible... but this is where you have to remember that sun sign doesn't say everything. I am a Taurus, but with my Mercury and Venus in Aries. I can have very strong Aries traits, and don't actually need my Aries man to bring excitement, challenge or fire. I already had that. In fact, I am more of a daredevil than he is, I am also less shy than him. And so I don't think that that's the main component of the attraction between Aries and Taurus.
With us, it has a lot to do with our Moon, Mercury and Venus signs being highly compatible - add to that that crazy Mars-Venus energy and you got yourself a very strong connection. I've heard of many Taurus-Aries attractions. I really think it might have something to do with the Mars-Venus thing.