I would love to know the answer to this question myself, as I am always attracted to Aries.
One thing I can say about Aries, from a Scorpio perspective, is that it's an instant attraction with us. We immediately want to copulate with Aries and vice versa. Though Scorpio is more cautious in their approach physically (unless they're in a social setting with booze around), Aries is very direct and responds warmly to Scorpio's seductive advances. The chemistry is crazy… in the beginning. Mars rules both signs, so conflict is natural between the two. However, I won't go too deep into that.
So why are Aries attracted to Scorpios?
I think it has to do with Scorpio's mysterious, stand-offish nature. Aries tend to be attracted to a-holes, jocky, physical, cool (on the surface), type people. However, Aries falls in love with depth (probably why they have relationship problems a lot, they settle for what's on the surface). When they see Scorpio respond in (what others interpret as) jerkish behavior or exert their power and influence, Aries wants to dominate that… Do Aries see a challenge or a trophy?
Beyond this, friendships can easily blur between platonic and situationship. The sexual chemistry is too strong to ignore, and the company between the two is fluid, especially when one needs to vent, the other seems to get it. Aries can be a calming presence for the hot-headed Scorpio and vice versa.
The chemistry between Scorpio and Aries is likely related to the planet they share, Mars.
It is highly recommended that you compare natal charts to get deeper insights between two individuals.