Attitude is a state of mind which, like thinking, can be passive/natural (i.e., effortless) or critical/active (i.e., requires effort). This is why is it much easier to have a negative attitude than a positive one.

To have a negative attitude, no effort is required. Whereas, a positive attitude requires effort, and many times great effort. And this effort is based on a given desire. Hence, attitude is a matter of desire