why are we so stressed?

Why are we all so stressed when we know that it's not good for us? Why when we try to meditate, relax, think positively, exercise? So why?!
Firstly stop being too harsh on yourself. Evolutionary and biologically we are wired in a way to ensure survival. Survival and protection are number one priority for our brain. And if we're closer to the stress response than the relax response we are more alert to fight or run away from any potential danger. But we have to remember that it is an instinctive response to ensure survival. The higher centres of our brain which allow us to think logically and analytically are switch off during stress response - no time to think, run or fight but do something now! This might have been helpful in the past when we lived in more danger times, when we had to be constantly alert and watch out for danger. But these days we live in a safe environment and most our stress giving problems would actually benefit from pausing and allowing ourselves the time to think them through before responding - not just reacting. 
We may not even notice that we live in a state of chronic stress and that our decision making is very much affected by it and in return our bad decisions are bringing more stress into our lives. 
Allowing yourself to relax and learning how to relax is immediately making you smarter and better at making the right decisions for you.  
So the good news is that it's not all our fault. It's not us but just one of the conditions of being a human. But we can work with it and consciously make the decision to relax whenever possible. 
Start with regular monthly massages and explore yoga to relax your body, mind and to get in touch with your Self in order to be able to response rather than to react to life's challenges.