Why is being too thin a bad thing?

If you are too thin and barely eating, your body doesn't have enough resources to function. When someone is too thin, they are probably not giving their bodies enough nutrients, so the body begins to break down: instead of relying on food for energy, it gets its energy from itself, meaning that the muscles and organs degrade so that the body can work. The heart is a muscle, so the heart is being stressed and becoming smaller and smaller, which can lead to serious complications, including a heart attack, and heart attacks are not a walk in the park! Similarly, the organs degrade too. If organs degrade, the body can't perform its functions: the liver stops working, the pancreas stops working, body functions aren't regulated. If the organs don't work, here again there are issues that may lead to serious consequences. That is why being too thin a bad thing: the body can't work and the person could be in serious trouble.

Being too thin is not a bad thing, neither is being not thin, everybody is unique and beautiful :)

Being too thin as in underweight is unhealthy, just as being overweight is unhealthy. But maintaining an unhealthy weight or even losing weight further could result in more dangerous health problems and even be fatal.

Because you aren't taking in all the vitamins, chemicals and other nutrition necessary to keep your body functioning. This will damage your immune system. Also, too litte fat reduces insulation, you have a lot less energy and therefore get things done less efficiently; and many organs are unprotected. For example, kidneys need a layer of fat surrounding them because they are very vulnerable and easy to damage.

I think as long as you are healthy, that "too thin" doesn't really exist. Staying healthy is important as we go through life, if you are on the thin side, that isn't a bad thing as long as you are healthy!

it depends whether you are naturally thin ,whether your slim or whether its unhealthily thin/ skinny

It isn't! As long as it's natural. If that's the way your body reacts to the healthy food you put in your body, then looking the way you do is beautiful. What's not natural is keeping your body from receiving the nutrients it needs, and therefore you temporarily loose weight. This form of 'too skinny' is extremely unhealthy and dangerous

It isn't so much that being too thin is bad, however being unhealthy is. If you are thin, but in good health mentally and physically then that is perfectly fine. If you are starving yourself, that raises need for concern as it is not healthy for you in any fashion.

Being too thin--by which I mean at an unhealthy weight for your body--is very, very dangerous. It can lead to some very serious health problems, many of which can be permanent even after you regain your health, or even to death. Anorexia nervousa is actually the most deadly mental illness because of how dangerous being 'too thin' is. It can do a lot of damage to many, many different areas of your body.