Why does St. John propose to Jane in Jane Eyre?

St. John is a stern, severe, rigidly self-disciplined, and religious person with a cold personality, although he is morally good and pure. He wants to marry Jane not because he is in love with her but because he thinks she is a capable worker who can help him with his planned missionary work in India. He tells her that he has observed her for the last ten months and has seen that she has the strength to do the work demanded of a missionary's wife.
When she says she would go with him as a sister and a helper but not as a wife, he says that if she comes at all she must come as his wife because anything else would be improper. He also states that he does not want her to come as a sister because that would leave her free to part from him at any time. He tells her:

I, too, do not want a sister: a sister might any day be taken from me. I want a wife: the sole helpmeet I can influence efficiently in life, and retain absolutely till death.

Jane refuses this loveless offer.