Accumulating excess fat tissue is bad for you. The mechanisms might derive largely from the presence of visceral fat tissue, which causes chronic inflammation and large changes in the operation of metabolism, leading to metabolic syndrome. It is also possible that other mechanisms related to nutrient sensing are at work, shifting portions of your biology into a fast-aging mode that evolved in some common ancestor to accelerate reproduction in times of plenty. Equally, being overweight tends to accompany sedentary behavior, and lack of exercise is very harmful to long-term health.
Regardless, it seems like a good idea to avoid becoming fat: the weight of evidence to show that it will harm your future health is heavy indeed, especially when it comes to cardiovascular disease:
Overweight and obesity likely cause myocardial infarction (MI) and ischemic heart disease (IHD); however, whether coexisting metabolic syndrome is a necessary condition is unknown. To test the hypothesis that overweight and obesity with and without metabolic syndrome are associated with increased risk of MI and IHD [we] examined 71,527 individuals from the Copenhagen General Population Study and categorized them according to body mass index (BMI) as normal weight, overweight, or obese and according to absence or presence of metabolic syndrome.
For MI, multivariable adjusted hazard ratios vs normal weight individuals without metabolic syndrome were 1.26 in overweight and 1.88 in obese individuals without metabolic syndrome and 1.39 in normal weight, 1.70 in overweight, and 2.33 in obese individuals with metabolic syndrome. For IHD, results were similar but attenuated. Among individuals both with and without metabolic syndrome there were increasing cumulative incidences of MI and IHD from normal weight through overweight to obese individuals.
These findings suggest that overweight and obesity are risk factors for MI and IHD regardless of the presence or absence of metabolic syndrome and that metabolic syndrome is no more valuable than BMI in identifying individuals at risk.