It has been suggested that optimistic self-assessments of unhealthy diets constitute a barrier to the promotion of healthier eating practices. In order to discuss possible reasons for such optimistic assessments, knowledge about the considerations underlying self-assessments of unhealthy diets is needed. The aim of this qualitative study is to explore this issue by comparing considerations underlying the assessments of people who overestimate the healthiness of their unhealthy diets with those of people who express more realistic assessments. Interviewees were recruited among those respondents to the Danish National Survey of Diet and Physical Activity 2011–2013, who had been accorded a low diet index score. A thematic analysis of qualitative interviews is undertaken (N = 16). When interviewees are asked to assess the healthiness of their diets, they draw upon their nutritional knowledge and their perceptions of healthy eating practices. However, these considerations tend to be overruled by more decisive criteria. Thus, diets are assessed as being not exactly healthy, but nevertheless healthy enough – so long as interviewees feel good. Moreover, a personal history of weight status and weight concerns emerge as decisive criteria in self-assessments. Those who experience problems in these areas tend to be realistic about the unhealthy character of their diets, while optimistic assessments appear to be linked to tendencies to perceive oneself as not being overweight, not having experienced weight gain or loss, or not being concerned about weight. This study concludes that decisive criteria in lay people's self-assessments of unhealthy diets – with regard to feeling and looking good – differ markedly from the criteria employed in food-based dietary guidelines. These broader criteria of assessment should be recognized by professionals engaged in planning health promotion strategies with reference to dietary health.