I'm going to the gym sick
do you like working out ?
Yes, it helps me have greater control over my health.
You are making your health situation worse by going to gym when you're sick. Not only that, you'll end up making other people sick too so you should stay home and take rest.
Why can't I go to enjoy myself?
Because by going out to enjoy yourself, you're going to make several more people enjoy themselves less.  Tradeoffs like these are how society functions!