there's a few things to it but mostly it's a choice and how you decide to live your life and the food you eat.
for those that say "it's genetic" it is in as far as your parents are fat lard asses and you end up being one too, so it's more social genetic than actually something in your dna. basically your parents didn't teach you to eat properly. my cousins for example a couple of them are fat, simply because my aunt would feed them french fries and fried food instead of healthy stuff, when we were younger my one cousin would refuse to eat veggies unless they were covered in gravy or some other sauce
as for the not being able to lose weight, that really comes down to what you're eating. if you want to lose weigth, cut out a lot of carbs and protein, ie meat and potatoes/rice, and keep active. you have to eat very healthy and you will drop weight, also cut out any fried foods, pop, drink pure juice and lots of fruit and veggies.
i got a bit of a belly but i do know how to get rid of it, i'm just being lazy. i just have to work at doing it.
now where the genetics comes in is where your fat gathers first, for me it's my belly, the rest of my body is mostly fat free. the only real exception really is females who typically gain fat in their hips first off but that's more for child rearing than anything else