St. John wants Jane to marry him, move to India and become a missionary with him. He believes she would make a good missionary wife because of her character and commitment. He has observed what a good friend Jane has been to his two sisters and he admires her many skills - as a teacher, governess, etc. He thinks she would make a good helpmate but he does not have the passionate character of Mr. Rochester. Jane has found him to be cold and suspects that while he admires her, he does not love her the way she wants to be loved.

St. John admits he has feelings for the wealthy Rosamond Oliver but he knows Rosamond would not even entertain the idea of becoming a missionary and moving to India. He therefore decides not to act on any feelings he has for her and asks Jane to marry him. Jane agrees, at first, to go to India with him, but not as his wife. She belives marriage to St. John would be a loveless one. She says:

Now I did not like this, reader. St. John was a good man; but I began to feel he had spoken truth of himself, when he said he was hard and cold. The humanities and amenities of life had no attraction for him—its peaceful enjoyments no charm. Literally, he lived only to aspire—after what was good and great, certainly: but still he would never rest, nor approve of others resting round him. As I looked at his lofty forehead, still and pale as a white stone—at his fine lineaments fixed in study—I comprehended all at once that he would hardly make a good husband: that it would be a trying thing to be his wife.

You can read the etext here on enotes. There is also an analysis to help you.