In Event Horizon, why was the doctor obsessed with hell?

Because he was promised to get his wife back

Many of the people on board of the Event Horizon is faced with their regrets and people they are no longer with them^*:

• Miller sees his subordinate who he has abandoned in fire
• Peters sees her son covered with horrible wounds
• Finally, Weir sees his wife, who has committed suicide, which asks him "to be together, forever"

Miller takes his vision as a hallucination, Peters is less convinced (we don't know if Peter's son is dead or alive, but she was really tempted to comfort "him"). Weird is faced with a choice: destroy the ship and whatever the chance he has to see his wife or accept the "things she wanted to show him". He chooses the latter, because "he has been so alone" since she has died.

So in summary: he is initially interested from a purely scientific point of view about the Event Horizon ship (since he has helped to build it), but then when faced with the hallucinations, he doesn't see them as horrible, but as a way to see his wife again.

^{*We don't know what Justin has seen, but it has put him first in a coma and later pushed to a suicide}