Is it bad to be a nice person but not have the energy to be nice and friendly on a consistent basis?

I'm just trying to rephrase my last question and statement. I guess I feel like I have to be perfect all the time just so that I won't get messed with by people. Do I have to be perfectly nice and friendly all the time? Or do I have the right to mess up sometimes? People just tend to take a lot of energy out of me when I'm dealing with them so I'm not always the most friendly at all times. So what am I supposed to do? Act like a mean jack 24/7? Or should I just try to do the best I can to be helpful and friendly? I really try but it seems people are giving me a hard time for not being perfect. And it's not like I give people a hard time about them or their beliefs. But maybe I slipped up somewhere with someone and didn't know it.