Being thin does not mean you are fit
This is a very important health tip that we should always keep in mind, even to the point of not bothering to weigh ourselves. The reason is that being thin does not always mean that you are you fit or healthy. This is because bodyweight is not the only thing that factors into being healthy. Many factors go beyond just you are outer appearance, and those are how strong your heart, lungs, and overall body and brain functions are. The other factor is muscle mass. If you are thin but lack in muscle, you may fit in the category of "skinny fat". This term comes from the fact that if you lack in muscle, regardless of your weight, your bodyweight must still be comprised of something, and often times that is in excess fat.
Thus don't worry too much about how thin you are but rather use more accurate measures of health and fitness. Such measures could be simply how well you feel, or how much weights you can lift, or even how fast or long you can run for. Improving on these factors can go a long way in improving and maintaining your overall health!