I believe being nice has a lot to do with state of mind and one's general perception of other people. Having empathy for issues that others are going through, and a general understanding that the other person is a real live person who may not see things the same way as you or might even just be having a bad day or something, can help.

I think one can be perceived as being nicer by being mindful of their tone. Ask yourself, "how will one feel when they read this?" It can be really easy to fall into the trap of saying things in a passive-aggressive way. It can even be fun at times to be sassy and loose with your language. But ideally it should be tailored based on your target audience (Wow, this sentence makes me realize that I've been in business school for too long. I really need to graduate). What was the general nature of the comment you are replying to? Is this the right time to be sassy, or would answering this question require a gentler approach? These are questions that you could ask yourself when commenting online.

I think a lot of it really boils down to one's own perception of themselves and life in general, as well as one's own attitudes and personality. If you're really serious about making efforts to be "nicer", then I would suggest looking into yourself and seeing what it is that is making you unhappy about yourself, if anything. In many cases our attitudes are just an expression of how we really feel inside. I know that for me personally, I tend to lash out a lot and be a jerk to people when I'm going though times of high stress and depression. During those times in my life, I really have to watch my tone and what I say in order to avoid being unnecessarily unpleasant with people.