Thought is not what inhabits a certain conduct and gives it its meaning; rather, it is what allows one to step back from this way of acting or reacting, to present it to oneself as an object of thought and to question it as to its meaning, its conditions, and its goals. Thought is freedom in relation to what one does, the motion by which one detaches from it, establishes it as an object, and reflects on it as a problem.
It's easy to get negative because you get beat down. You go through a few disappointments and it's easy to stay in that negative frame of mind. Choosing to be positive and having a grateful attitude is a whole cliche, but your attitude is going to determine how you're going to live your life.