You can and should be thin. — This is a very dangerous assumption.
Let's break it down into 2 parts, 1) You can be thin; 2) You should be thin.
1) You CAN be thin.
The reality is that this is not true for everyone. Bodies come in all sizes. It's genetically predetermined, just like your shoe size.
A person may temporarily be able to lose weight through dieting, but this goes so drastically against our biology that most of the time the weight is regained. This pattern of "weight cycling" (losing and gaining) actually results in weight gain over time, and can increase a person's set-point weight range, meaning that their body will gravitate towards a higher weight as a form of self-protection.
2) You SHOULD be thin.
In our society we have associated thinness with: success, control, intelligence, willpower, happiness…
Who wouldn't want these things?
And if that's what thinness will do for you, then it's easy to see why we are placing so much value on it.
The problem is, thinness doesn't bring these things. It's an empty promise.
Bodies are beautiful and normal at all shapes and sizes. No matter what your body looks like, it deserves to be taken care of. There is no "right" size to be. Treat your body with the respect it deserves by giving it nourishment, movement, and rest.