Lord Capulet is very much a man of his time and class. As a father and an aristocrat he expects total loyalty from those beneath him, both family members and servants alike. In keeping with tradition he's arranged a marriage between his daughter and a suitable young nobleman, Paris. It is his sincere wish that Juliet should go ahead with the marriage and he fully expects her to comply. For aristocratic families at that time, marriages were strategic political alliances rather than love matches. Juliet, however, is deeply in love—but with Romeo, not Paris.
Being both headstrong and in love, Juliet defies her father's wishes. As one can imagine, Capulet's absolutely livid at her insubordination. He cannot believe she could be so ungrateful after all he's done for her:
Hang thee, young baggage! Disobedient wretch! I tell thee what: get thee to church o' Thursday, Or never after look me in the face. Speak not. Reply not. Do not answer me. My fingers itch.—Wife, we scarce thought us blest That God had lent us but this only child, But now I see this one is one too much And that we have a curse in having her. Out on her, hilding! (act 3 scene 5)
Capulet's so angry at his daughter's willful disobedience that his fingers itch; in other words, he feels like hitting her. Though his reaction may seem over-the-top, Capulet is acting entirely within the bounds of what was considered acceptable for someone in his position. In those days, a father was expected to be lord and master in his own home, especially if he was a real lord like Capulet. As such, he was deemed to have complete moral and legal control over his entire family. In refusing to marry Paris, Juliet isn't just defying her father, she's defying social convention, the glue that holds society together. Capulet understands this all too well, which is why he reacts in the way he does.