Now this is not saying that you cannot be nice to someone because that is just plain stupid. Kindness and niceness are good friends most of the time. But next time you are about to say something or NOT say something ask yourself is it because you are worried about their feelings or their heart. If it is their feelings then maybe figure out a different way to say it so that their hearts can be put in the right place and their feelings spared. Christians shouldn't be worried about having to always be nice - Christ wasn't always nice and God surely isn't. God knows EXACTLY how much food and water your body needs to sustain life and if he needs to withhold that in order to bring you closer to him, then he will do that. It isn't nice, but it gets the job done and your heart in the right place - closer to him. He uses all sorts of things - we see it all the time in every day life, finances, marriages, relationships - go ahead, look around with fresh eyes and see how God uses every day things for his purposes. So many people want proof - what they want is physical manifestation of something that they don't believe in anyway because the proof is in front of them already and they have excuses or explanations for those.
So although I may not be the nicest person, I am kind. I am striving to be more nice or at least more gentle because Christ WAS gentle and he is my greatest hero, then it is Tim Tebow and Sailor Moon.