Like they don't get to be kids. Like they have to be responsible too much of the time. When they feel sick 'yet another time', they fear it will be BAD this time. They could feel important for something besides illness. Plans have to be cancelled at the last minute because of illness or treatments.
Sleepovers are for others.
It's often out of the question that we go because of meds and treatments and things, but everyone else gets to go. Later, when kids talk about the sleepover at school, imagine how we feel. They have to make up homework while other kids are playing. They miss out on organized sports because coaches don't understand the illness. The whole class has to leave a field trip early because a teacher didn't think it was important to bring an inhaler for one of them.