your body has this thing called metabolism, it is different for each person, but it is how many calories your body gets rid of before it starts gaining weight, this can be altered slightly when taking into account: your general eating habits and exercise. but mostly it is genetics. 
weight doesn't mean too much without a height as well.  So if you are very tall, 142 may be underweight. but if you are shorter, you may be slightly overweight for your height.
hope this helps you with your mystery.

It's weird, I eat cookies, candy, ice cream and all sort of those things all the time, but I am still skinny

your body has this thing called metabolism, it is different for each person, but it is how many calories your body gets rid of before it starts gaining weight, this can be altered slightly when taking into account: your general eating habits and exercise. but mostly it is genetics. 

weight doesn't mean too much without a height as well.  So if you are very tall, 142 may be underweight. but if you are shorter, you may be slightly overweight for your height.

hope this helps you with your mysteryi.

I wasn't quite being literal with this post x3 bit yes, I am tall, thus, very skinny.