What reason does Paris give for Lord Capulet's decision to move up the wedding?

At the beginning of Act Four, Scene 1, Paris visits Friar Lawrence's cell to discuss the upcoming wedding. Paris tells Friar Lawrence that Lord Capulet insists on moving the wedding to Thursday. Paris explains to Friar Lawrence that he hasn't had a chance to discuss romance with Juliet, but her father is trying to rush the wedding to stop his daughter from crying. Lord Capulet believes that Juliet would benefit from having a partner during the difficult time, who would prevent her from shedding tears over Tybalt. She is still grieving over Tybalt's death, and Lord Capulet feels that Paris will help her get over the loss, which is why he is moving the wedding date up. Paris specifically tells Friar Lawrence,

"Now, sir, her father counts it dangerous / That she do give her sorrow so much sway, / And in his wisdom hastes our marriage / To stop the inundation of her tears—" (Shakespeare, 4.1.9-12).

After listening to Paris's explanation of why Lord Capulet feels that it is necessary to move the wedding to Thursday, Friar Lawrence says to himself that he wishes he didn't know the reason why Paris and Juliet's wedding should be slowed down.

The answer to your question can be found in act 4, scene 1 of this play. We find Paris in the middle of a conversation with Friar Laurence about the forthcoming wedding between Paris and Juliet which has now been moved, we hear, to "Thursday." The Friar thinks this is very short notice, but Paris says that "my father Capulet will have it so" and comments that he himself has no desire to argue about it.
The Friar suggests it might be prudent to wait if Paris does not "know the lady's mind," but Paris explains that Juliet has been in such despair over Tybalt's death that her father "counts it dangerous / That she doth give her sorrow so much sway." That is, Paris's understanding is that Juliet is so upset about Tybalt that her father thinks a wedding would be the best thing to take her mind off it ("to stop the inundation of her tears"). He says that Juliet has been "too much minded by herself alone" and that a wedding will bring her back into contact with other people.