Why can't I just have a fast metabolism?

Breakfast:
1 egg
English Muffin
Butter (1 tsp)

Midmorning snack:
Nectarine

Lunch:
McDonalds Southwest Grilled Chicken Salad
Diet Coke

So I didn't pack anything for lunch so when noon rolled around, I was getting quite hungry. One of the girls at work was thinking McDonalds and I automatically thought "Great! I can have the salad." Well, until another coworker talked about getting a Big Mac. And Fries. And I automatically wanted the same thing (well except chicken nuggets until of a Big Mac). So on the drive to pick up the food I had this conversation with myself:

"I want Chicken Nuggets. Fries sound so good."

"You can't Stephanie. You'll have to blog about it. You don't want to blog that you are still fat and eating fatness are you?"

"What if I dont blog about today? Whats the harm in eating fatty mcdonalds?"

"Now you're lying to your readers and yourself. Thats how you got here, remember?"

"Ugh. Why must you be right?"

So I pulled into the drive-thru ordered my salad and now currently consuming that instead of the Chicken Nuggest and Fries.

Why must we desire the fatty goodness that is fried, baked and loaded with not-so-good-for-us ingredients?

Why can't I just have a fast metabolism?