Why is being too thin a bad thing?

When you push your body too far it will have an adverse effect on your health. Whether that's eating too much or too little, not exercising enough or exercising excessively, you're putting a lot of strain on your system and that can lead to complications such as bone and muscle weakness, lower levels of organ function and lowered immune system efficiency. Of course, some people are naturally thin or naturally large, and that's fine. You can be healthy at a wide range of different shapes and sizes, and no one should be commenting on that or shaming you for it. It's the methods some people use to achieve these sizes that can be unhealthy, not the sizes themselves.

Well I'm pretty thin myself I don't think it's a bad thing but some people do, it could mean your not eating how you should not looking after yourself right so that is a bad thing. But some people can't help being thin some people are just like it

It's bad for your health, mental and physical. But this really depends on your definition of 'too thin'. There is a BMI that is appropriate for all heights and ages. I would say if you are at or just slightly under your BMI that isn't a problem, But if you are more than a few numbers under your BMI that is unhealthy mentally and physically for you.

We have to change the idea of being "thin" and "fat" to being "unhealthy" and "healthy". Everyone has a different genetic make- up, Some people a short, tall, broad, petite. It's who we are. So yes, some people may be naturally thin. Being too thin is only a concern when there are negative thoughts or unhealthy eating habits associated with it.