If you have a mental illness such as anxiety or depression you might not want to tell others due to the unavoidable stigma regarding mental illness.
I am rather open regarding my anxiety and depression but, that is because I have a safe environment. I know not everybody has a safe environment to express themselves as I have received my fair share if judgment regarding the topic but at the end of the day it's a personal choice.
I felt sharing what was going on with me with my close colleagues would not only help me but also help them understand why I sometimes just get down and sad when nothing obvious happened. It also helps me feel safe in my work environment because they know how I am and hence they take that into account which makes anxiety attacks at work easier to handle. Obviously, everyone differs and I am only sharing my experience on the topic. Knowing people I see 8 hours a day at least understand me, gives me peace of mind and also the courage to open up.
Now I do know that not everyone will experience sharing their mental illness at work the same. I also encountered lost of glaring looks and nasty remarks. People lash out at stuff they do not understand and they say hurtful things. Hard as it is if you are willing to open up you need to be ready for some shots being fired at you. There will always be someone who says they experienced the same thing and they're just fine, good for them! We weren't born to react, think and feel the same as everyone around us. Opening up is hard, it took me a while and I got a lot of judgment from people really close to me but it also helped me identify who really cares for me.
We all have different stories and just because I reached a point where I can share my story doesn't mean everyone can. You are allowed to tell the world the pain it caused you and you're also allowed to keep it to yourself.