Because we know what the outcome will be. Sure we'll feel sore the next day, but after our muscles build back up we will be able to do more than we did before. We have to break our muscles down to build them back up bigger.
This same idea goes for charisma. Your charm is not just an endless flow of awesomeness continually emanating from your body (wouldn't that be cool though?). It's something you can consciously turn on and off because you only have a limited amount of supply. I've been reading a lot that you only have a set amount of willpower to use throughout the day. If you are on a diet and constantly smell that box of donuts your co-worker brought in, it's going to take a lot of energy to not eat one of those donuts. It will physically and mentally drain you. BUT, continual exposure to and resistance of the donuts will build up this supply of willpower. Sure, in the beginning you will be exhausted, but each time it will get easier and easier.
I've been noticing the same thing when I'm trying to be charismatic. I'll get really tired after only a couple hours. I'll sometimes go home thinking "Man this stuff is exhausting, is it even worth it?"
The answer has to be yes. I'm just not used to being so outgoing for extended periods of time. I have to break down my mental walls to build up my endurance.
That being said, i probably am overdoing it a little bit. This improv stuff is going STRAIGHT to my head. I find myself improvising during almost every conversation. The other day I told my mom that I was getting a tattoo of a dragon on my left ass cheek. She didn't freak, but she certainly was not pleased.
I'm going to try to tone it down a bit, but not too much. I want to get used to being mentally fatigued, so much so that it doesn't affect my charisma anymore. I need to get to the point where I'm not thinking about all the things that go into a charismatic personality and it just becomes second nature. I need to let the subconscious take over, and to do that I have to teach it through my conscious mind.
I just ordered this book "Mind Over Mood" which pretty much teaches you exactly that - how to continually repeat things to yourself using your conscious mind so that you can train your subconscious mind to think in more beneficial ways. Mad excited to start that, I'll let you know how it is!