Getting sick is normal. Everyone catches the occasional cold or virus and eventually recovers, but being chronically ill is different. It's like one day you caught the flu and never recovered. After awhile, it becomes difficult to talk about, either you become ashamed because you may not be the same person anymore, or you might not want others to think of you differently. There are so many reasons why people don't talk about their illness.
One reason is the sadness. Talking about being ill around others makes them want to comfort you or pity you. Whoever you open up to could be having an amazing day, everything is perfect and the sun is shinning in every aspect of their life. You don't want to ruin your friend's awesome day with how much pain you're in, so most spoonies will keep quiet about whats bothering them and move on.
It also reminds you of the person you can no longer be. You start to mourn your old self, whomever that may have been. For me, I mourn the active, fit dancer who could dance all day long, go to school, and hang out with friends and only be a normal tired. Now, I can barely get through the day. I mourn the athletic body that I had and how confident it made me feel. I miss the old me, the one who's pains came from her career and not a disease. So, if I am talking about my illness, I try to speak positively or with humor, keeping away from the death of the person I used to be.
Opening up about your hardships and triumphs is difficult. Sometimes it's brushed off as you complaining, or someone says "Me too! I get that all the time!". If you're saying that to me, I think you need to go to a doctor because most of the things I talk about are not normal. Those who open up about their illness don't need you to feel like you have to understand, we mostly just want a hug and an "I love you", or "is there anything I can do to help?".
It can be incredibly embarrassing to talk about your illness. A lot of weird things can happen to your body when you're sick, things that you never want to speak of ever again. There are hospital moments that no one knows about except for those who witnessed them because they're so terrible. You're body is revolting against you when you're ill and it loves to show how rebelious it can be. Opening up about that can be awkward and weird, so if someone does let you in on one of those secrets please don't make the situation worse. They trusted you with something scary, don't make it so they won't share ever again.
There is a certain fear of rejection or of being abandoned. Illnesses make you feel isolated, like you're the only person in the room who feels this different. I've had people leave because of my illnesses, and others who have only wiggled their way closer to me. It's a hit or miss when you open up about your illness. You fear that your friend will find you odd or too much to handle now. That they'll no longer invite you over, want to be with you, or even want to talk to you. You become different from them, and if you're young like I am, that can scare people away. No one wants to be abandoned, especially when their body has already given up on them.
If someone opens up to you about their illness there are many ways you can make it easier for them. Let them finish their sentences, ask questions about their illness and how it works, and lastly, offer them any support that you can give them. Offering your help makes it easier to accept it and shows that you care about them.