A personal theory on Newtypes. This isn't canonical, but I think it explains what's going on pretty nicely.

The Newtype phenomenon is caused by physical or mental trauma resulting in an inability to communicate with others normally. A series of short case studies:

Amuro Ray is a solitary youth whose antisocial tendencies are caused by a cold mother and absentee father. He has no normal human relationships, and does not begin to find his place in the world until after the development of his powerful Newtype abilities.

Char Aznable is a self-described loner who has taken on a series of false identities in order to avenge his parents' murders and promote his father's ideology. The only common thread among his many female companions is that they are all Newtypes - he never opens up to an Oldtype, and his constant need to battle Amuro can be seen as a desire to interact with someone whom he understands.

Camille Vidan is a violent young man with a lightning temper - his civilian hobbies seem limited to mechanical tinkering (like Amuro) and judo, which he regularly skips. He can't stand his own name and, though he always disliked his parents, was shocked when they were killed in front of him. Like Amuro, he only starts to figure out where he sits in relation to others once his Newtype powers start to manifest.

Jared Messa is an arrogant Oldtype bully, apparently uninterested in human relationships, until the trauma of seeing partner after partner killed by the crew of the Ahgama forces him to develop into a Newtype. Even then, he remains callous enough to use nerve gas on a civilian colony in an effort to end the war.

Paptimus Scirocco, though we know little about his past, has spent years in comparitive solitude commanding his small crew back and forth to Jupiter. His only interaction with others is psychological manipulation, at which he excels due to his Newtype abilities.

Haman Khan is only 19 years old and the sole dictator of an insular society of rebels, ruling as a regent to the child-queen. She is, succinctly, a bitch, and though we don't get much direct information on her background, we can use her youth and canonical timelines to deduce that she started whoring herself out for political influence at age fourteen.

Quess Paraya. Her last name’s a homonym for pariah, for God’s sake! 'Nuff said.

All of the miscellaneous mini-Newtypes (Katz Kobayashi, Hathaway Noah, Sayla Mass) are war orphans.

All of the artificial Newtypes, from "strengthened humans" like Four Murasame to the "cyber-enhanced" like Gyunei Guss are mentally ill, ranging from neurotic to completely nuts. Rosamia Badam, for instance, thinks that Camille Vidan is her brother (he isn’t) and is constantly worried that the sky is going to fall on her. I think that the loss of memories, mental illness, and inability to get along with anyone without becoming totally obsessed aren’t side effects of the artificial Newtype program - they’re essential to it! Newtype-ism is probably multifactoral (having something to do with genetics, environment, and psychology), but I’m convinced that there will never be a well-adjusted Newtype, because maladjustment is a prerequisite for the condition. Newtypes, for one reason or another, are unable to connect with people in the usual way, and so reach out in new ways.

Another small theory on the subject: if Newtype psyco-waves aren’t electromagnetic, what are they? I suspect that Newtype powers are transmitted by gravitons – that’d explain why Newtypes never develop on Earth, and often talk about their souls being “weighed down by gravity.” If psyco-waves are transmitted by gravitons, then on Earth there might be too many ambient gravitons floating around for proto-Newtypes to interpret incoming psyco-wave signals correctly – too much mental white noise. Once they’ve developed their abilities in space, however, they can then return to Earth and sort out authentic psyco-wave signals from the graviton morass, but not without some additional effort. This is often interpreted as a “weight” on the soul, much as the presence of hostile Newtypes is often described as “pressure.”