Excess visceral fat tissue and other side-effects of the sort of high-calorie, low-exercise lifestyle required to pack on the fat will do you great harm in the long term. Getting fat is a choice is for the vast majority of people, a choice made again and again day after day by deciding to eat more calories and skip exercise in favor of other activities. For 99.9% of the audience here: you're not special, and there's nothing in your genes that's making it noticeably easier to gain weight or harder to lose it. But if you let yourself become fat, just as so many other people are doing in this age of comparative wealth and low-cost calories, then it will come back to bite you.
See this paper, for example:
In the second half of the 20th century it became obvious that a relentless increase in diabetes type 2 (DM) affecting the economically affluent countries, is gradually afflicting also the developing world. This review juxtaposes the threat that the DM epidemic represents to mankind, with the astonishing recent discoveries on the role of obesity and of the body fat in this metabolic disorder.
Presently, the highest prevalence of DM is in Saudi Arabia, a country deep in riches generated by its oil wells. DM is very high, in over 10% of adults in the USA, Switzerland and Austria. Prevalence is low in Norway, China and in Iceland. Predictions of epidemiologists for the first third of the 21st century claim up to 2.5 times increase in DM in the Middle East, Sub-Saharan Africa, India, rest of Asia and in the Latin America. In China the number of patients with DM will double but in the economically advanced countries that experienced rise in DM in the 20th century, the increase will be only about 50%. Remarkably, a lowest increase in DM is expected in the countries that formerly belonged to the Soviet political space.
Increasing urbanization, aging populations, obesity, and falling levels of physical activity are all contributing to the rise of DM worldwide. The main cause of DM pandemic is growing prevalence of obesity, in Europe and in the Latin America. In the North America obesity is considered to be responsible for 90% of DM in females. Male obesity is associated with DM slightly less, at 70-80% in the European Union and in the US.
Thin, healthy people aren't thin and healthy because they have magic genes. They're thin and healthy because they are eating less and exercising more than the fat folk. This isn't rocket science, and it does have an impact on the bottom line of your life expectancy.
Ginter E, & Simko V (2010). Diabetes type 2 pandemic in 21st century. Bratislavske lekarske listy, 111 (3), 134-7 PMID: 20437822