While it sounds simple, it's unfortunately not for the vast majority of client's I've seen. Some have limited opportunity for mastery due to socioeconomic status, too many demands on their time, and/or multiple psychosocial stressors. Others do not experience themselves as competent, regardless of their ability levels, due to low self-esteem and/or perfectionism. Some have areas of competence but are impeded from pursuing them by low energy and motivation, high anxiety, etc. All of the above are missing out on a key resource for recovery and longer-term health.
Sometimes clients' symptoms actually offer them some sense of mastery. For example, weight loss creates a feeling of success in clients with eating disorders, and following particular rituals perfectly may offer a feeling of competence to someone with OCD. In these cases, mastery is a double-edged sword: its power as a reinforcer may strengthen the symptoms we're working with clients to change. As a result, it becomes even more crucial to help clients find something other than the symptom through which they can experience mastery.
The need for mastery applies to therapists, too. However, while work is a core source of mastery for many people, therapy perhaps offers fewer opportunities to experience mastery than other fields might. The work we do is definitely not concrete, rarely visible or measurable, and change happens slowly. While there are those breakthrough moments that are exhilarating to witness and leave us with the feeling that we're making a difference, there are also many moments where the results are much harder to recognize. Many of the therapist I know compensate by finding mastery in other areas (e.g., hobbies or family roles), and a surprising number even find comfort in the "busy work" of the job - paperwork, data entry, even adding labels to charts.
Is mastery an issue for your clients? Do you address it in your work? If so, how? Where do you find your own sense of master?