In Star Trek: Generations we see Picard’s and Kirk’s idea of a perfect life, even though the former only stayed in the Nexus for a few minutes.
What do you think Soran’s perfect existence in the Nexus was? Was it ever mentioned in the E.U.?
StarTrek.com's official page for Soran describes him as using the nexus "as an escapist portal for returning his dead family to virtual life with him"
In the film 'Star Trek : Generations' Guinan describes life within the nexus as
"like being inside joy. As if joy was something tangible ...and you could wrap yourself in it like a blanket".
Later, during his first encounter with Soran, Picard attempts to reason with him by highlighting the amorality of his actions;
"PICARD: What you're about to do Soran, is no different from when the Borg destroyed your world. They killed millions too, including your wife and children.
SORAN: Nice try. You know, there was a time when I wouldn't hurt a fly. Then the Borg came. And they showed me that if there is one constant in this whole universe it's death.
Afterwards I began to realise it didn't really matter. We're all going to die sometime. It's just a question of how and when. You will too, Captain. Aren't you beginning to feel time gaining on you?
It's like a predator. It's stalking you. Oh, you can try and outrun it with doctors, medicines, new technologies, but in the end time is going to hunt you down and make the kill.
PICARD: It's our mortality that defines us, Soran. It's part of the truth of our existence.
SORAN: What if I told you I've found a new truth.
PICARD: The Nexus?
SORAN: Time has no meaning there. The predator has no teeth."
Soran is clearly attempting to enter the nexus in order to avoid his own mortality and to return to a time (even if virtual) when his family was still alive.
On top of that, the nexus creates an artificial sense of 'joy' that is so attractive that Guinan thinks that anyone would be tempted to stay, regardless of their mental strength.
If Soran's experience is anything like Picard and Kirk's then he can expect to be shown a mish-mash of scenarios that are intended to represent his various longings and regrets made whole.
The movie mentions that Soran has lost his family (as far as I recall during a Borg attack - yup, seems correct). I assume he hoped to be reunited with them (actually the linked article specifically says so, but I'm no expert on Star Trek Canon Levels).