Lord Capulet thinks that Juliet is grieving too hard for the death of her cousin, and has decided that a hasty marriage to Paris is the cure for it. This is ironic, because when Paris first approached Capulet about marrying Juliet, Capulet is opposed to Juliet getting married too quickly, and insist on them waiting a couple of years. His mood is also depressed and emotional. His emotions are on edge, and one could come to the conclusion that he is seeking relief from his own grief as well as Juliet's. Based on the very violent nature of his response to her refusal to marry Paris, it is clear that Capulet it being controlled by his emotions.
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.
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.
I tell you, he that can lay hold of her
Shall have the chinks.