I think it was because he was injected with some sort of poison or chemical when he was a child soldier. Another reason is that the patriots used as a test subject for surgery for exoskeletal enhancements leaving him as a cyborg, risking his normal life.
Well, that sucks for him 
Well, he doesn't have to sleep, can deflect machine gun fire by reflex and gets to toss ginormous robots around like rag dolls. And he gets paid to do so. It's a livin'. 
Sure, if he doesn't mind no longer able to do basic Human activities, like eating, sleeping, drinking, and a long list of "other" personal bodily functions. Don't get me wrong; being a super human cyborg is awesome, but still I'd prefer a more consistent life of normalcy, than an exciting one of uncertainty.
Very valid point. Well, he believed he had to become a cyborg so that he can fight battles no one else would dare to fight. He's been through a lot of suffering, most notably his guilt of many past sins. He doesn't want anyone to suffer as he did and as you can see, he's fully commited to that cause. And besides, someone who has spent his whole life on the battlefield, the definition of an uncertain and exiting experience, ever since he was six years old would have many problems with a more peaceful life style. In fact, it may be impossible. Maybe that the poor man doesn't think himself as a human anymore...
Im kinda to late (3 years to be exact) but after playing though MGR like a million time now I learned how Raiden ticks(or at least have a general understanding) At the start of MGR he gave up his ´human body´ to commit fully to his Revenge and thus made himself 95% machine with his new body/armor. He thinks what he is doing is right and justified because he avenges all deaths caused by Desperado and doesnt care why or who they are - as he calls "I´m the Reaper everyone involved will die." Later he realises that this was all a good excuse to keep his past in check(his childhood/jack the ripper time) and admits that he just wants them dead, he doesnt care about the law or what other think about him(including Rose, Bladewolf etc.).
At the end of Metal Gear Rising, Raiden realises that he never cared to be the ´hero´ like Snake was, he just wanted more blood spilled and made up good excuses for it. In the very last cutscene Raiden explains to his former chef that he wont return because he - as he puts it. "I´ve got my own war to fight´"
Many people hated this change in Raiden´s overall lore because, for them, Raiden becoming pyscho is dumb. But it absolutly makes sense, he fought his entire life starting back he was just a child. Already at the age of 10 having a kill count over 1000. Even in MGR Raiden says that it only was half the truth that he left Rose and Little John, it just wasn´t because he made money by doing the exact same thing that he used to(protecting, killing etc.) but also because he enjoys war since its the only thing he know´s for real. Also he is strongly afraid to return to Rose because he knows that he is way to fcked up now(even more so than in MGS4) and also because he, like i´ve already said, only really knows how to kill, not how to be a parent.
About the "think about himself" part, Raiden doesn´t consider himself human anymore, no. He said he´s just a machine made to kill, he let his new body be constructed only for the single purpose of assassations and general killing. That means he doesnt feel anything anymore except pain. For example, Raiden doesnt feel the way we do anymore, it all feels the same, cold and empty. He doesnt need to eat(although he can) nor drink. And since his body is now literally a living armory he doesn´t have his ´private parts´ he doesnt have to visit toilets or feels ´lust´.
(english isn´t my main language so their could be some miss spellings and of course, that i posted this reply 3 years to late ^^)
At the end of Metal Gear Rising, Raiden realises that he never cared to be the ´hero´ like Snake was, he just wanted more blood spilled and made up good excuses for it. In the very last cutscene Raiden explains to his former chef that he wont return because he - as he puts it. "I´ve got my own war to fight´"
Many people hated this change in Raiden´s overall lore because, for them, Raiden becoming pyscho is dumb. But it absolutly makes sense, he fought his entire life starting back he was just a child. Already at the age of 10 having a kill count over 1000. Even in MGR Raiden says that it only was half the truth that he left Rose and Little John, it just wasn´t because he made money by doing the exact same thing that he used to(protecting, killing etc.) but also because he enjoys war since its the only thing he know´s for real. Also he is strongly afraid to return to Rose because he knows that he is way to fcked up now(even more so than in MGS4) and also because he, like i´ve already said, only really knows how to kill, not how to be a parent.
About the "think about himself" part, Raiden doesn´t consider himself human anymore, no. He said he´s just a machine made to kill, he let his new body be constructed only for the single purpose of assassations and general killing. That means he doesnt feel anything anymore except pain. For example, Raiden doesnt feel the way we do anymore, it all feels the same, cold and empty. He doesnt need to eat(although he can) nor drink. And since his body is now literally a living armory he doesn´t have his ´private parts´ he doesnt have to visit toilets or feels ´lust´.
(english isn´t my main language so their could be some miss spellings and of course, that i posted this reply 3 years to late ^^)