Obesity is not a disease, but it has many causes. For most people, yes it is a question of too much and poor choice of food and drink, but there are a significant number of people who are overweight for reasons that are not easily within their control. some have it as a side effect of another illness - polycystic ovary disease, or as a side effect of the medicaments they need to take; steroids, for example, and there are also a number whose overweight is genetic.
I had a fascinating interview between Jim Al-Khalili and a woman doctor whose name eludes me in the R4 programme 'The Life Scientific'. She researches the role genes play in obesity and it is quite clear that for some people there is a strong genetic influence.
The best way to tell the difference is to look at pictures of an obese persons family, if photographs taken 50 or more years ago, show that some members were obese, even though you know they led busy active lives (housework and washing were hard manual work then), then there may be a genetic cause, but if everyone is slim, then it is less likely.