The next morning, Mr. Collins asks for a private meeting with Elizabeth. The rest of the family scrambles out of the room. When they are alone, Mr. Collins explains in detail his two main reasons for seeking marriage: all clergymen should marry, and Lady Catherine told him to do it. He details his income and stresses his future wife's association with Lady Catherine.
Mr. Collins wants to marry because of outside pressures. As he explains his reason for marrying, it becomes clear that his wife will be merely an ornament in the "respectable" life he's creating for himself.
Elizabeth interrupts to decline, but Mr. Collins responds that women will typically reject an offer two or three times. He goes on to say that Elizabeth should accept him because she's not likely ever to get a better offer. Elizabeth insists that she's serious, that she and Mr. Collins could not make each other happy. He doesn't understand. In exasperation, she leaves the room.
Mr. Collins makes a valid point: based on her family and situation, Elizabeth isn't likely to get a better offer. By declining his offer, she is giving up her family's best hope to hold on to their home. Even so, Elizabeth believes too strongly in love to agree to marry a man whom she doesn't even like.