Why is Lord Capulet determined to marry Juliet to Paris?

Capulet is likely happy that such a well-qualified suitor is already asking after his young daughter.  Count Paris is a kinsman of Prince Escalus, and therefore his status in Verona would be high.  In this way, it is an advantageous match for Juliet and would raise the status of her family, the Capulets, as well.  However, none of this is made explicit.  

The only really explicitly stated reason that Capulet wants Juliet to marry...

Capulet is likely happy that such a well-qualified suitor is already asking after his young daughter.  Count Paris is a kinsman of Prince Escalus, and therefore his status in Verona would be high.  In this way, it is an advantageous match for Juliet and would raise the status of her family, the Capulets, as well.  However, none of this is made explicit.  

The only really explicitly stated reason that Capulet wants Juliet to marry Paris is that he wants to help Juliet get over the loss of her cousin, Tybalt.  Lady Capulet tells Juliet, "thou hast a careful father, child, / One who, to put thee from thy heaviness, / Hath sorted out a sudden day of joy [...]" (3.5.112-114).  In other words, Lady Capulet says that Juliet's father, out of concern and love for Juliet, has figured out a way to temper Juliet's sadness with an unexpected happy occasion.  Thus, it sounds as though his intentions are quite good.  

However, when Juliet rejects Paris, it seems as though Capulet gets incredibly angry, not because he was so attached to Paris, but because Juliet has dared to disobey him.  After Juliet has lied and made them believe that she has seen the error of her ways, Capulet says that his "heart is wondrous light / Since this same wayward girl is so reclaimed" (4.2.49).  He is most happy that his daughter has returned to her former obedience, not necessarily because she's about to marry Paris.