In Act III, Scene 5, why does Lord Capulet want Juliet to get married quickly? What is his mood?

While I totally agree with the above answer, I would add two more reasons why Lord and Lady Capulet want Juliet to marry Paris.

First, he is physically very attractive. The Nurse says he is "a man of wax" and Lady Capulet says, "Verona's summer hath not such a flower." Lady Capulet goes on to further describe Paris's attractiveness in Act I, Scene 3:

Read o’er the volume of young Paris’ face,
And find delight writ there with beauty’s pen.
Examine every married lineament
And see how one another lends content,
And what obscured in this fair volume lies
Find written in the margent of his eyes.

For Lady Capulet and especially for the Nurse, physical charms are very important in choosing a mate. The Nurse is equally impressed by Romeo and says in Act II, Scene 5:

Though his face be better than any man’s, yet his leg
excels all men’s, and for a hand and a foot and a
body, though they be not to be talked on, yet they
are past compare.

Secondly, the match between Count Paris and Juliet can be seen as socially and politically expedient. The fact he is a Count obviously means he is quite wealthy. Juliet's family too is very rich, as indicated by the Nurse's words to Romeo in Act I, Scene 5:

I tell you, he that can lay hold of her
Shall have the chinks.

The marriage would also be beneficial to Lord Capulet politically since Count Paris is a relative of the Prince who rules Verona. In reality, a marriage between Juliet and Paris makes perfect sense. They are of the same class and close in age, although Paris's age is never specified. Romeo would also be a good match socially and financially if it were not for the bitter feud (definitely part of Friar Laurence's thought process in marrying the doomed youngsters).