Your parents don't understand your point of view. To them, happiness is having a goal accomplished. And their goal seems to be to raise you as best they can, so they are probably a little overbearing from time to time.
Now, not being ambitious is fine, and living a normal and happy life is even better, but your parents don't see it that way. Their greatest fear seems to be you waking up one day (say, in your mid 40's) and asking yourself, "What have I done with my life?" and then going out and buying a sports car you cannot afford and divorcing your wife for a younger girl.
This is more common than one would think. It's called "the mid life crisis" and most male members of society suffer through it. From the poorest of the poor, to the richest of the rich, the saddest of the sad, and the happiest of the happy, there will come a point in everyone's life where they look back and realize that they could have done more. And yes, it will happen to you as well. The danger in being unambitious is that when this time comes, even if you are very successful and happy, you might look back and realize that you could have done so much more, which will rob you of your happiness.
If you would like to prevent this from happening and at the same time please your parents, come up with a simple goal. Ask yourself, "Where would I like to work?" Your answer could be your ambition. If you strive to get the job, you likely will. There will be obstacles, sure, but overcoming those will grant you a surprising amount of satisfaction. Then, when you finally do get the job that you desire, during your mid-life crisis you will be able to look back and realize that you did accomplish something.
Believe it or not, as a youth I was unambitious as well. To this day, I still cannot decide what path I should take in life. My overall ambition is a rather vague one as well. "Gain enough power to be able to ease the suffering of humanity" is my personal goal. Mind you, this may seem like a selfless goal, but I am not a selfless person. The reason I have this goal is because with less suffering, there is more productivity. People who aren't suffering ask questions, and seek answers. If I were able to stop global hunger, massacres, and poverty then perhaps there would be enough scientists world wide to prolong my own life span and delay my decay, as it were.
This ambition itself is the cause of much anguish for me, simply because I cannot decide how to best go about this. Do I go into politics? Do I become the head of an international corporation and use my money to do it? Do I gather all the world leaders together in one room and yell at them until they finally get their act together? I am constantly plagued by these questions, and so I know that I am not as happy as I could be.
However, I believe that living a life without pain is dreaming, not living. To me, life is the most difficult game to play. Dark Souls has nothing on the massive multiplayer mode that our own reality has. Pain, in and of itself, is not a bad thing either. Pain is like salt. You add it to a meal to make the meal more satisfying, but if you add too much then you ruin your meal.
I'm going to stop here. This post is already really long, and you are an intelligent enough person to be able to use what's already here to your advantage.