Why is it said that women should always be obedient. Why is society so harsh on them and their dreams?

Man and Woman have equal dignity and therefore one is not inferior or superior to the other. 

Obedience is “a moral virtue that inclines the will to comply with the will of another who has the right to command.” According to St. Thomas Aquinas, obedience is a special virtue, related to justice, by which we pay our superiors a special honour which is due to them by reason of their excellence of authority, namely, the fulfilling of their precepts or commands.” Therefore, a person who is rooted in obedience submits his will to the one who possesses legitimate authority over him.

The Catechism of the Catholic Church addresses the virtue of obedience as a duty. “The duty of obedience requires all to give due honour to authority and to treat those who are charged to exercise it with respect, and, insofar as it is deserved, with gratitude and good-will.” (1900)

Obedience teaches us to be humble and the spirit of obedience helps to shed our ego and pride. So that makes obedience a necessary virtue for both man and woman. We are all called to be obedient to those in authority, most especially to God.

Your question leads me to assume that in your case someone in authority may have clipped your desires and dreams and you are frustrated about it. And you feel if you were a boy, things would have worked out favourably. 

Sometimes those in authority might want you to obey them because they feel or fear that something untoward might happen to you. They do this out of their love and concern for you. I do understand the frustration especially when you think you are mature and smart enough to handle situations, but those in authority fail to see it. I would say, please be patient. The time will come when they will be convinced that you are capable of handling things and will permit you to pursue your desires and dreams provided they are good for you and for others. Sometimes our desires and dreams could become absolutely selfish and we completely ignore the good of others. A family or a society grows and matures only when the common good is sought by all. So it is important that your dreams and desires are good enough.

Now when it comes to man and woman, though they are equal in dignity, they are not similar. Our bodily strength, mental and emotional capabilities differ. And because of our fallen nature, both man and woman tend to have disordered or warped desires which one might want to fulfil at the expense of the other. This becomes harmful and dangerous especially for those who are weak. So it is important that we not only learn to defend ourselves but also learn to avoid situations. Because women could fall victim to the disordered male dominance and arrogance, parents and those in authority are likely to take necessary precautions. This is because they want the best for you and because you are dear to them. 

When we are very young, we are in the learning stages of life. It is not necessary that we understand everything. We must understand that parents and those in authority are God given. They are given to us by God to guide and lead us. Obedience is an act of faith.  It takes a little patience and humility to submit to authority because

And as a final note, if you feel the authority is acting uncharitably and against your greater good, I would encourage you to discern the situation again with the help of the scriptures and with the Church’s teachings. You may consult matured elders, priests or religious. With their help you could prayerfully discern the next step. But do everything with great humility and respect. God bless you.