It's an interesting state of disorder. Weight gain is caused by a surplus of calories. How your body reacts to that is gnetically determined...that includes how well leptin works in your body (this system is broken by prolongued obesity, will find the paper later). There's also factors such as how well your body can increase thermogenesis (again, not a huge margin but another system that breaks long term...result is berths ultimately). Environment is a factor to an extent, bad parental influence on food awareness and abundance of poor quality food (starbucks, mcdicks, etc) combined with the wrecked biology by the time a person is making independent choices then leads to a body not capable of much in the way of exercise intensity...and given that we now generally get everywhere by sitting in a box on some number of wheels, caloric expenditure by way of lifestyle craters. It's cause and resolution is multifactoral...I.e eat less, move more is as simple as can be in describing the fix, but in the 3/6/12 months it takes to fix someone's weight problem you're having to manage various things.