Act 3: Why does Lord Capulet agree to let Paris marry Juliet?

Capulet thinks Tybalt's death has made his daughter sad beyond reproach,

Things have fall'n out, sir, so unluckily
That we have had no time to move our daughter.
Look you, she lov'd her kinsman Tybalt dearly,
And so did I.

Capulet wants Juliet to be happy so he agrees to the marriage.