what are mr.collins's reasons for marriage? how does elizabeth feel about the proposal? why does she ask mr.collins not to think of her as a "lady" the way he describes her?

His reason are those of convenience. It doesn't really matter which of the sisters he marries, in fact any of them will do. The same goes for the women...... if any of the girls marry Collins, the women in the family will have a sense and form of financial security. Mr Collins comes out ahead simply because he enters his ministry with a wife by his side.