There is this very bold theory about Mitsu and all the other non-homo dancers that get the lead dancing roles and after some time get kicked out from the team.
If you look close at Mitsu during some performances (which is hard, because camera often avoids him, except for AT05), you'll see how focused he was on Ayu.
Even if the dance didn't required him looking at Ayu, he did that, way too often. Don't bash me, but I think he had some feelings for her and that might be the reason he got kicked out.
He was somehow Shuya's predecessor, he was allowed to do things others couldn't at that time, like touching Ayu's thigs (in my name's woman pv), he was the person who laid his head on her legs during Fairyland PV, he had the lead role in my name's woman during Arena Tour 2005 intro and performance, there's much more of that.
Maybe he wanted sth he couldn't get and Ayu got rid of him and since then she decided to give all the leading role to Shuya (since he's homo and she would feel safe about him). Now there's also Maro, but still only these two, neither Zin, neither Subaru get the hotter scenes and dance moves.
Back then there was also Naoto, who also has been kicked right after he got his lead roles. Something is wrong with that.
Very short before Mitsu's dissappearance he was behaving strange, kinda lost his energy, wasn't focused at all. In CDL 05-06, during winding road Ayu starts singing changed lyrics, all the dancers start to laugh but Mitsu somehow wakes up and smile only after a while.
But that's just my theory, it may be totally rubbish, but that's how I feel. Mitsu was my favourite dancer, he has awesome body and captivating gaze.
I don't want to be seen baseless so here are some screencaps. See how often he looks at Ayu although everybody else is concentrated on dancing.
http://img17.imageshack.us/gal.php?g...swomanpv03.jpg
Feel free to discuss but don't throw meat, it's just a theory