It's a slippery slope that takes a lot of time with lots of small incremental weight increases that are barely noticeable.
At first, you just notice clothes getting a bit tighter. You find yourself buying the next size up when you come to buy new clothes. The ones that were tight eventually get too tight to wear, or they naturally deteriorate due to time and get thrown out anyway.
It's not easy to lose weight. Only people who've never had a weight problem think it's easy. There are usually reasons for weight gain, often lack of time or inconvenient to exercise, often lack of options for healthier food, sometimes lack of funds to pay for better food options. There is also mental and behaviour issues in play that usually need professional help to resolve. Telling someone with a food behaviour problem to eat less is just like glibly telling a drug user to stop taking drugs or a smoker to stop smoking.
I was obese, mostly due to unhealthy relationship with food stemming from early childhood. I spent all my teen years obese. I lost a shed load of weight when I was around 20 because I started working, got more control over my life (less parental influence), got new hobbies, started voluntary work, etc. I lost around 6 stones! A few years later, I changed jobs, to one with a much longer commute, very stressful, long hours, etc - I had to give up some hobbies and voluntary work and returned to my teen year habits of comfort eating. Put on 10 stones over around 8 years! Incrementally, didn't really notice at first, because I'd just buy a size bigger every time I needed new replacement clothes. It took me 20 years to lose it all, that was due to a heathier more active life, changing jobs, marrying, having a child, reducing to part time working, taking new hobbies again. Now I've been at a healthy weight for a few years, mostly because I'm not stressed with work anymore, have time to exercise daily, have time/money to cook healthy meals etc.