Risa and Satoshi Reasons
1. Risa doesn't act like the other girls.
Hmmm...this is a little hard to explain. Risa doesn't oggle Satoshi like the others do. (I don't know why, tho. ^^) These other girls have failed to get Satoshi's attention despite their faraway idolizing and/or their love letters yet Risa is the only girl to have any verbal contact with him.
2. Satoshi tried to return the white ribbon
Sounds a little lame but here's the story, Daisuke had his white ribbon fall out of his pants. Satoshi, picking it up, saw Risa running away from the stairwell where Daisuke dropped it and thought it was Risa's. From that point on, he went around looking for her. The fact that he even gave the effort of looking for her must mean something. (Or not. *lol*)
3. Their common obsession.
Heh...this is also a con but we'll deal with that later. ^^;; Yep, Risa and Satoshi share an obsession and that's Dark. (0_o) Because of this, they can, despite their differing personalities, relate to each other at times and will probably see each other more because of him. A LOT more if Risa keeps on setting off traps and Satoshi has to save her. -_-;;
4. Satoshi is attracted to Risa
Did you do a double take at this reason? huh huh? j/k. Actually, this is more speculation than fact but I'll put it up since it doesn't quite go in any other section. Recently, Satoshi was left with an unconscious Risa on the beach. He walks over to her and he kinda touched her hair. I dunno, I thought it was an affectionate (tho, a little scary 0_o) gesture. It's enough for one to see some sort of attraction coming from him.
5. They make a great couple
OK...don't kill me for this one but I honestly think they'd make a great couple. Lots of arguing and misunderstandings at first but I can see these two working things out in the end. Satoshi's calm and controlled life could use a little chaos from Risa's outgoing personality, and I can also see Risa being able to deal with Satoshi's random angsty/depressive behaviour. That and if Satoshi had any problems, Risa would probably find out what was wrong by pestering it out of him. ha ha...
More to come... I'm actually doing this on a spur of the moment so I apologise if these reasons seem ridiculous and such. Give me time to plan this out a little and maybe they might seem a little more valid. ^^