How does someone let themselves become obese?!

some people don't care, or are not motivated.
Depression is also another cause.
Some people really just love food too much.
And some people are just too lazy.

Its all about the inner personality

Perhaps they are more motivated to eat more rather than exercise or eat less. Furthermore, they just may not care about eating healthily. However, a lot of obese people did not simply "let" themselves get that way. In some cases, there are medical conditions that cause them to gain weight easily, and lose it with great difficulty. That's not to say that they are always eating healthily or attempting to lose weight.

Common sense.

It's usually just genes. Like how some people can eat whatever they want and never get fat. Some people get fatter even if they don't eat as much as other people. That's the most common reason. Otherwise it's probably an eating disorder. I doubt anyone would forcibly eat until they were about the explode everyday in order to get that fat without having an eating disorder.

ask Eric Cartman that I dunno. LoL
I'm just joking.

People just want it (the food) cause they have depression or something mental and it eases the painn.
Or people just keep pushing bounderees and saying like "I will lose 50 pounds next week" or "I can do it later" so they get bigger and bigger and they never achieved their goal. :( sad aint it? Haha

I am NOT fat nor was nor ever will be.

It's not like obese people are obese on purpose and they ENJOY being obese. Sometimes it's a hereditary thing. It all comes down to their self control and their self determination to exercise.

with a lot of food?

just with a lot a lot of food....

a eating disorder, depression, ignorance about nutrition, little exercise

Food is the number one addiction.