"My reason for marrying are, first, that I think it a right thing for every clergyman in easy circumstances (like myself) to set the example of matrimony in his parish.  Secondly, that I am convinced will add very greatly to my happiness; and thirdly – which perhaps I ought to have mentioned earlier, that it is the particular a advice and recommendation of the very noble lady whom I have the honour of calling patroness."( p. 73,74 )

    From this paragraph we can find that his desire for marrying was not out of the sincere, honest love toward a lady (unlike Darcy), but based on that universally acknowledged truth.  Besides, his purpose of marrying was to please Lady Catherine, so that he could maintain the close relationship with her.  And that would be a great benefit to the promotion of his career and status.  He chose Elizabeth just because he believed Lady Catherine would like her.  He would marry a lady who would be accepted and appreciated by Lady Catherine, whether they know each other well or not.  It was he that had to get along with his future wife in the rest of his life, not Lady Catherine; It was his marriage, not Lady Catherine’s, so how could he made this significant decision only based on Lady Catherine's preference but without his own opinion.

There are some other examples to prove what I said above: 
1) "and your wit and vivacity I think must be accepted to her, especially when tempered with the silence and respect which her rank will inevitably excite.  Thus much for my generation intention in favour of matrimony."( p. 74 )
2) "Were it certain that Lady Catherine would think so, but I cannot imagine that her ladyship would at all disapprove of you.  And you may be certain that when I have the honour of seeing her again I shall speak in the highest terms of your modesty, economy and other amiable qualifications."( p.75 )

    "It does not appear to me that my hand is unworthy of your acceptance, or that the establishment I can offer would be any other than highly desirable.  My situation in life, my connections with the family of de Bourgh, and my relationship to your own, are circumstances highly in my favour." (p.75,76 )  The paragraph above shows he had confidence that once he gave a lady his proposal, it was impossible for any girl to reject him.  And that was why he considered it was the delicacy of the female character when Elizabeth rejected his proposal.

    As a rector of a parish, Mr. Collins had a certain status in his society.  Though his social class was not so high , it is above the average at least.  Therefore, he should be polite and humility, in particular, he was a rector.  He had to be stricter with his conduct and manner.  It is also the same concept in his marriage: he needs to a gentlewoman who has qualifications of being modesty, amiable, useful and active.  In addition, burdening the title of rector, Mr. Collins was conservative, and dared not to break the routine --- marriage is a market, and the women are its merchandise.