This article is from the ER FAQ, by Rose Cooper email@example.com with numerous contributions by others.
During the first season, Susan dated one of the psych
department doctors, Div Cvetic. However, Div gradually became
chronically depressed for reasons that are still a mystery. His
depression manifested itself as an increasing hostility toward his
patients. He failed to show up for Thanksgiving dinner at Susan`s
place, in "ER Confidential" , and at the end of that episode, we
see a troubled-looking Div darting on foot in the middle of downtown
traffic. This is the last we ever see of him. Two episodes later,
in "The Gift" , Susan arrives home (they were living together)
to find that Div had disappeared, and moved all of his belongings
out, without any notice or warning.
Later in the year (in "Love Among the Ruins", ), Susan
discovers a recent snapshot of Div in a dating service book owned by
a taxi driver who runs the dating service on the side. Div is
apparently one of the cabbie`s success stories; he tells Susan that
thanks to his service, Div is happily married to a woman to owns a
chain of mortuaries. This seems to present pretty firm evidence that
wherever Div is, he`s living a life far removed from the show`s
characters, and will therefore likely not return any time soon.