Lord Capulet thinks this because Juliet, after she has agreed to the Friar's scheme to drink the sleeping poison, tells her father that she has had a change of heart and will assent to marry Paris. This takes place in Act IV, Scene 2, and Juliet begs for her father's forgiveness, saying that her visit to the Friar taught her to:
...repent the sin
Of disobedient opposition
To you and your behests, and am enjoin'd(20)
By holy Laurence to fall prostrate here
To beg your pardon. Pardon, I beseech you!
Henceforward I am ever rul'd by you.
She even describes her meeting with Paris at the Friar's, a rather tense, frosty affair in reality, as the moment when she proclaimed her love for the young suitor:
I met the youthful lord at Laurence’ cell
And gave him what becomed love I might,
Not stepping o'er the bounds of modesty.
This, of course, is part of the scheme, a ruse designed to get her parents off her back. With her parents preoccupied with the wedding, she will drink the poison and fall into the two-day sleep as the Friar has planned. Juliet reveals that she has become a decisive, brave woman.