In Act 3, Scene 4, Lord Capulet very rashly offers Juliet's hand in marriage to Count Paris. He says that he is willing to make a "desperate tender of [Juliet's] love," meaning make a "desperate offer of [Juliet's] love" (III.iv.12-13; eNotes). He says he is so certain "she will be rul'd" by him and consent to marry Paris, that he rashly promises Paris his bride before she agrees. This decision was particularly rash because a marriage in the middle ages was only legal with the mutual consent of both parties ("Late Medieval Canon Law on Marriage"). If one partner refused to consent to the marriage, the marriage would be made void, which is one reason why Friar Laurence shows so much concern at the haste of the marriage and the fact that Paris does not know how Juliet feels about the marriage, as we see in his lines addressed to Paris, "You say you do not know the lady's mind / Uneven is the course; I like it not" (IV.i.4-5).
Beyond being rash, Capulet's sudden decision also contradicted his earlier decision that Juliet is too young to marry, and he does not want her marrying until she is at least 16. However, the reasons behind his sudden, rash decision can be easily understood in the next scene. As Juliet's mother explains, Lord Capulet is very concerned about the depth of Juliet's grief over what he thinks is Tybalt's death. He sees that such bitter, prolonged grief is unhealthy and hopes the marriage will give Juliet a happy distraction from her grief.