This is one of the mysteries of the Sandman storyline. I have a story about it eversince but the picture is not pretty:

The old Destruction raped Delight thus turning her into Delirium. That destruction is quite powerful that he can destroy memories and traces of the event from Destinys book. It also is one of the reasons why the new Destruction abandoned the position forever (though Delirium has already forgiven him, thus seeking for his return).

Any theories about it to share?

I believe there was only ever one Destruction. Despair was the only other one that died aside from Dream.

I heard a theory once that she slowly morphed into Delirium when people began looking for Delight through recreational drug use, therefore she grew to reflect a trippy, strung out aspect of Delight that resulted from that. Another one I heard once was that she just simply grew up. How could anybody keep innocent Delight forever? Someday, we all have to grow up, and that obviously didn't do well for Delight.

There was a thread about it some time ago, it's probably lost in later pages...

There's an actual sort of explanation of why Delight turned into Delirium in one of the Sandmans or in one of the Deaths. I'm sure someone remembers it better. I think it involved a girl that was abused by her father and either Delirium is part of her, or they are somehow connected. But that was for certain the event that triggered the change.

And it had nothing to do with Destruction if I remember it well.

I think it's in Brief Lives that there's a flashback (could it be through Destiny's pages flipping?) and we see Delight as she's starting to feel the change... And she feels the change, but she can't help it (therefore, it's gradual). It also says, in there or in some other part of the Sandman series) that it happened in the early stage sof humanity (so I wouldn't make the connection to recreational drug use).

Sorry for being vague, I don't have the comics with me. At any rate, the ide aI got was, when the universe was new and everything shiny, there was Delight (as the humans discovered everything and loved it and learned stuff), but at some point that turned sour -I always made this connection to the retreat of the Feary World from our world, which started with the use of cold iron. That is, at some point, the humans stopped being dleighted with their lot and world and started being nasty and trying to get advantage of things, and they got their pleasure in from "darker2 things, so Delight as we knew it was lost forever, but we can still glimpse her when we're in an altered state of mind (drugs, alcohol, lack of sleep... you know, that moment when we're going mad AND at the same time there's something great and happy about the world...).

I think she was raped, but not by one of her fellow endless.

Reasons:

1. in Seasons of Mists we meet her and she says she doesnt like to be touched. People who were raped are very nervous around people who want to get all touchy feely. she says some boy tried to "kiss" her but she didnt want him to. She also says she was almost married once.

2. in "Endless Nights" the Delirium story follows a girl who had been raped by a man wearing a fish mask who goes "to deep inside" of deliriums realm. Delirium also becomes lost in her own realm. The girl that was raped and a handfull of other crazies go on a rescue mission (I realized, not to long ago they were sent by Daniel) and bring her back. The girl that had been raped ends up having blue and green eyes and a fish thing. I think its a subtle way of saying this isnt deliriums story, but in a way it is.

3. there are a couple references though out the series where she isnt just different, shes sad and hurt. You can tell she isnt Delight anymore, part of that soul is gone. What happened was tragic.

4. In destructions story in Endless Nights the girl that Destruction impresses says that delirium looks like she had her heart broken. The man she was supposed to marry did something to her.

and lastly. I think it helps explain why the endless are not allowed to love mortals. The Endless are very big concepts and if a mortal can change one with such an act, it's better to keep them apart. I mean look at dream. He ended up KILLING himself in a way, because of love.

Here's another thread that dabbles with it.

Wikipedia says;A young girl whose form changes the most frequently of any of the Endless, based on the random fluctuations of her temperament. She tends to have wild and colorful hair and eccentric, mismatched clothes. Her only permanent defining characteristic is that one of her eyes is green and the other blue, but even those sometimes switch between left and right, or, if she absolutely has to become as orderly as possible, become the same color. She was once Delight, but some traumatic event caused her to shift into her current role. Audiences are to infer that she changed also as a result of the world's shifting from taking delight in simple pleasures to the more modern state of pursuing altered consciousness. Her symbol is an abstract, shapeless blob of colors.

In the old thread someone mentioned Adam & Eve and the Fall of Man. Someone also mentioned the possibility of a specific time in history like Destruction leaving during the Enlightenment. Alot of people have also mentioned rape.

I don't think it was the 'Fall' of man rather than the Rise of man that transformed her. The specific time was when apes achevied consciousness. Similar to the forbidden fruit, this is the time when we aquired sentient knowledge, arts, abstract thinking. Before this time all animals lived in a state of perpetual ignorant bliss.

In a way Delight was tainted by consciouness and the new ability to perceive suffering and mortality and feel confusion and doubt. This taint could be akin to 'rape' if you want, it's what happened when humans got too close, possibily uninvited. The burden of conscionsness came crashing in and hurt her...but it also developed naive Delight into maturity. She's very happy in her own way, Delight is still in there but she's no longer pure.

To me Delerium is the human condition of not knowing who and why we are, but then she's also a celebration of our existence and abilites. With Despair she's the most humanly insightful and sensitive of the Endless.