I can't tell you how many times I've had clients come into the gym in a bad mood, tired, sore, you name it; but for whatever reason just not feeling very good. And those very same people, more often than not, leave the gym feeling way better than when they arrived.
A big reason why this occurs is because of a hormone called endorphins, aka the "happy hormone". Exercise triggers the release of endorphins which are known to reduce the perception of pain and create positive feelings in the body.
So the next time you're considering skipping a workout because you don't feel very good, take a second to think again. More often than not, you will walk away from that workout feeling better than when you started.