You’re quite right! Factors like quality of life and personal satisfaction are very subjective measures. But studies show a direct link between patient satisfaction and improved outcomes from treatment so these elements are important.
Patient satisfaction may be what each person feels is important. In order to get some kind of measure, researchers devise a survey to include questions important to their study.
For example patients may be asked to rate the healthcare they received. They may be asked how difficult it was to see a specialist or if their care was delayed. Some patients rate their satisfaction with care strictly on the basis of how well the doctor listened or how much time the healthcare professional spent with them.
Sometimes patients are just given a scale from zero (no satisfaction) to 10 (complete satisfaction). Using this scale, they can say the number that best matches their own satisfaction however they define it.