一 So long as Mizuki feels in control, Mizuki is comfortable, in a mental sense. Obviously, he is most comfortable when he is obviously and safely in control, like when he's at tennis practice. Everyone on the team knows he's in control and no one is going to question his authority. Mizuki, unlike Fuji, who likes to push things to see how they'll come out with little investment in the actual end product, likes to manipulate means to an end; he likes to see that he can control all the variables and make them turn out the way he wants them. On the other hand, when Mizuki willingly gives up control, such as seducing people of power, this is still considered being in control, because, again, though he's the one pushed on his back, he's serving his ends; in the morning, he'll have that person in his control, and so he's not really giving up control at all.
二 The above point is generally considered null in the case of the Fuji kyoudai. Going into his relationship with Fuji Yuuta, Mizuki thought he wanted control, just as in all the above cases, but he realizes he was wrong. At the end of the St. Rudolph match against Seigaku in the regional tournament, when Mizuki allows Yuuta freedom to make his own decisions, Yuuta stays. This is the defining moment, the turning point in their relationship-- before this, Mizuki was dealing with Yuuta as he would with anyone, he wanted to control him entirely. I don't even believe Yuuta was really anything special to him besides a stepping stone to Fuji Syuusuke, honestly. But then Yuuta did what no one did-- he said no, and then he stayed. From this point, Mizuki's interests shift; he's still drawn in by Syuusuke in that obsessive, white whale sort of way, but on an emotional level, he's attached to Yuuta. He wants to figure Yuuta out, and he certainly still likes getting his way (he's a high maintenance boyfriend XD), but he's open to Yuuta in ways he isn't open to anyone else.
三 Before I go on to Fuji Syuusuke, this brings me to a point that I feel the need to interject-- Mizuki seems to be terrified of being hurt. He makes himself necessary to the team with data tennis (rather than actual ability; Mizuki's good but not that good) and manipulates people into doing what he wants. He paints a hard shell of lacquer around himself and doesn't seem to let anyone on the inside. No one sees Mizuki unmade up and undone, no one sees Mizuki hurt or lonely. Mizuki is Perfect. The brief tantrum he throws after the loss to Seigaku kind of shocks me, actually; it's the only time we really see Mizuki come undone. My theory on this is that Mizuki suffered some harsh loss early in childhood-- a friend moved away and didn't ever write, or else a pet or relative died. He seems unwilling or incapable or letting anyone inside his shell for fear that it might hurt him. He needs to be in control, or else he might get left. Yuuta seems to be the only one who has breached this protective barrier. If Yuuta left him, I honestly think Mizuki might go mad.
四 Back to Fuji Syuusuke. I don't really understand the basis for Mizuki's obsession with Syuusuke, so, unfortunately, I'm going to kind of gloss over this, but there are a few points I have in mind. Mizuki is obsessed with Syuusuke in a very unhealthy manner and wants to see Syuusuke come undone. They're very similar in this way-- they both have an outer shell and are very calculating when it comes to encounters of their sort. Mizuki does differ from Syuusuke on a few vital points. Where Syuusuke is bored all the time and plays tennis to amuse himself even though he never even bothers to play seriously, Mizuki is hell bent on winning (control again, it's all in control), and where Syuusuke isn't really conscious of having emotions most of the time (he's awfully confused every time Taka does something nice for him with no ulterior motive...) Mizuki is overly conscious of them. It sort of adds to the mess that Syuusuke is Mizuki's main competition in Yuuta's attention and affections, and though he likes to remain in control of himself in his games with Syuusuke, when it comes to Yuuta, Mizuki will get temperamental and snappish and lose his cool exterior.
五 Mizuki has a very acute sense of shame. When he loses control utterly is generally the only time he feels shame, but when he does, it's all-consuming. He loses his carefully-crafted exterior and has fits when he's ashamed. This is also the only time he cries. Mizuki is not one to use tears to get what he wants; he uses sex and his looks and money and whatever else, but he doesn't cry (after all, crying would ruin his perfect face and perfect skin and perfect eyes). The only time Mizuki cries is when he's very ashamed. Having Yuuta see him lose control would be the most shameful of all to Mizuki; because he actually cares and Yuuta actually cares, to have Yuuta see his defenses torn away would be the most painful of all (whereas Syuusuke seeing him in this state [and most likely causing it XD] is upsetting but is only losing the game, whereas Yuuta seeing is getting hit in the face with the tennis ball and losing the match.)
These are the points I've come to as of right now. There may or may not be more. Feel free to comment and argue with me or ask me things or whatever. ♥