Why do some people lose weight easier than others?

WOw!Iso many people wanting to be anorexic today.I pray,live,exercise,EAT healthy foods,just so I can put on lot’s of pounds.But the process is slow and needs a lot of commitment.A lot of people don’t symphatize with me when I tell them I am too skinny,they keep on telling me that they want to be skinny and that even water makes them fat.I can eat a whole mcdonalds pack gain weight the next day and then lose weight immediately.It’s like I keep having to eat through out the day.That helps a lot though!The point is I seriously HATE being skinny!I JUST HATE IT!I am 20 pounds under my normal weight.And every day is a fight to gain those pounds!!! I am just wandering why is it that some people lose weight easier than others???