BENJAMIN: Because I'm interested, Mrs. Robinson. Now what was your major subject at college?
MRS. ROBINSON: Art.
BENJAMIN: But I thought you…I guess you kind of lost interest in it over the years, then.
MRS. ROBINSON: Kind of.
Mrs. Robinson has just told Ben she doesn't know anything about art, and now he finds out it was her college major. She never got to develop her identity as an artist; her unplanned pregnancy and loveless marriage disrupted her life.
Mike Nichols described Mrs. Robinson to Anne Bancroft as someone who was angry with herself for giving up who she really was for wealth and security. She said she remembered this as she was filming this scene. (Source) Benjamin doesn't want to follow her example. But he already has, in a way—whatever he studied in college means nothing to him at this point, and he didn't come out of school with a sense of who he is and what he wants to be. His reasons are different, but the result's the same.