Social incentives such as cooperation and competition are found to motivate users in pervasive fitness applications. This work investigates how social incentives work for individuals with obesity and diabetes. We used a mobile fitness application called HealthyTogether as an experimental platform, which allows dyads to achieve fitness goals together and compete in an online community. We conducted a four-week study with 16 obese and diabetic patients who used HealthyTogether to exercise with a buddy. Results show that participants exercised more with social incentives compared with their baseline. Collaborating with buddies to compete in a community was reported as motivating for dyads exercising with strong ties. Social interactions could be demotivating between dyads who did not know each other well. Finally, it is crucial to consider patients technical literacy when designing behavior-changing technologies.