Lord Capulet is trying to arrange a marriage for his daughter, because she has come of age to marry. In the days when Shakespeare was writing, women married as soon as they were capable of bearing children. Lifespan was shorter than today, so adult life began much sooner.
Lord Capulet believes that he is helping Juliet to start her own life and move on from her mourning of Tybalt. He is thinking of her best interests.
Juliet is already married to Romeo, she is therefore in a very difficult situation. She cannot explain to her father that she married the son of his hated enemy. So she decides to kill herself. It creates the catalyst that starts the whole deception rolling.
Lord Capulet believes that Juliet's crying fits on Monday night and Tuesday morning are due to Tybalt's untimely demise. What he doesn't know is that she is crying over her brand new husband, Romeo's, banishment for killing Tybalt. Fearing that she might do something desperate because she "immoderately weeps" for her cousin, her father thinks that marrying her to Paris is just the ticket to cheer her up. After a stubborn refusal from Juliet (who didn't bother to tell her father that she is already married), Lord Capulet is angered that his daughter would dare to take such a tone with him, and he then demands that she marry Paris on Thursday or it's out to the street she goes. Juliet turns to her friend the Nurse for comfort, and the Nurse tells her to stop crying over Romeo and go ahead and marry Paris. As a last resort, Juliet begs the Friar for help, and if he can't help her, she has just the dagger to do so. He concocts his plan to tell her father that she will marry Paris and then fake her death; however, like "the best laid schemes of mice and men", the plan goes awry. Papa Capulet is so happy with Juliet's change of heart that he moves the wedding from Thursday to Wednesday, altering the plan. Romeo never gets the news that Juliet is only faking death, leading him to kill himself. When Juliet wakes up to a dead hubby, she follows suit with his dagger.