Why does Mr Collins speak so highly of Lady Catherine de Bourgh?
Mr. Collins speaks highly of Lady Catherine de Bourgh because of three things. The first reason is his mentality. The second is his employment. The third is his aspirations.
As to Collins' mentality, he is one of those deluded individuals who by sheer application and belief in hard work have attained a sought after goal. He has feelings of accomplishment and superiority that are founded on vanity and conceit. Also, while rising through the ranks of education, he entertained a sense of moral superiority because he did not give in to the wild insobriety of other collegians.
As to his employment, Collins considers it a rare achievement to have been employed as parish clergyman by a personage so wealthy and influential as Lady Catherine de Bourgh. He also feels that he and she share the same sense of moral certitude. This means that he may add pride to other flaws of vanity and conceit because he feels like he is really an equal of lady Catherine de Bourgh, who is very important in her part of England.
As to his aspirations, he wants a wife. He believes that speaking in glowing terms about the single biggest accomplishment and benefactress in his short and puffed-up life will win the admiration of a potential bride. Since he believes that he and Lady Catherine de Bourgh are alike and he her equal, he doesn't see how anyone in there right mind could fail to think him the luckiest and most blessed man in England, which, in his mind, should go a long way to getting him a wife.