Questioning??

Why do my parents accept everything about me? Why don't they scold me or slap me when I behave rudely, when I argue loudly, when I sulk and take out my irritation on them, when I trouble them about every small problem that I have even though I am old enough to argue that I should be allowed to make my own decisions regarding sex, alcohol, smoking et al?
Why do I always complain about them having too much expectations from me and burdening me with their ambitions? Why do I complain when they make certain decisions about my life that I feel I should have the right to decide?
Don't they have the right to do so? Aren't they allowed to keep even a certain amount of control or have a right on us. What do they feel when they are shouted upon by something(one) they took care of since the time it (he/she) had no friggin idea about the world it had come into?
Is it so unjustified on their part to have some expectations from us when they put in a lifetime in making us what we are today? I mean we have expectations from a program (I am a programmer :D ) after we have spent 2 hours on it with a clear objective in mind. They spend two decades of their life (more for others) and they aren't supposed to have even the slightest hope of how it turns out. They sacrifice their ambitions, dreams and desires; instead they turn us into all their aspirations but we feel extraordinarily burdened by their goals that they wish we should achieve.

I am fed like I have returned from a POW camp whenever I go home. The first missed call on my cell is of theirs when I am being bashed up by my friends on my birthday. All my calls end with " lots of love". What have I done to actually deserve this?? Am I still doing something to actually reciprocate it?
Am I grateful for what they have done for me so far? Am I ungrateful or just another person in this world? Maybe my parents are like that towards their parents. Is it a cycle of thanklessness that is continuing that more knowledgeable people term as 'nature's law' and it happens in all species (almost) of the world? I don't know and I don't care. I question my identity, my existence. The quotes "I think therefore I am" or "I am therefore I think" become meaningless. I would not have been and not been able to think. I would not have been such a mentally functioning person to feel that I have outgrown my dependence on them or think about writing this blog..