"It's genetic." I've heard the excuse before. Except it isn't solely an excuse--there is some true to it. Some people just have to work harder than others. Some have fast metabolisms and some have slow metabolisms.
That's not to say you can't alter your body in an advantagous way.
Habits also play an effect. Studies I have read show that one is more likely to be overweight if they have overweight friends/family. My only friend is 330lbs. and her husband is 500lbs.!!
My father did not teach me good healthy eating habits. Growing up with a large inactive father was not helpful with my own personal weight battle.
In High School I lost 30lbs. and weighed around 170lbs. I was in shape for two years before gaining weight again
--medications for my depression. Now I need to lose a lot of weight before a gay guy will even think about checking me out. Some guys are just plain shallow
--but others, like myself, want to date "in-shape, heathly guys." Being healthy--not smoking, low alcohol consumption, not getting drunk, not doing drugs, eating healthy, excersising, etc.--is a quality I look for in a guy, or rather, will look for in a guy when I'm physically ready to date. Guys how are healthy generally care about themselves. Of course there are exceptions to every rule