My personal philosophy is to be kind, even when others annoy you and even when you're having such a bad day that all you want to do is sulk in bed. There are plenty of people who, when upset, bring others down with them by choosing to focus on the negative, and while I believe feeling the negative emotions you experience is healthy, I also believe that you should move forward after feeling those emotions.
Choosing to be kind is much harder than it seems because we're human and feel annoyed, angry, and sad sometimes. Some days, we just want to lay in bed all day and think about everything that went wrong that day, and other times, we take out our frustrations on others. For me, letting my bad mood affect others has become apparent in my life -- I notice when it happens, and I always feel terrible about myself after the day is over. So why don't I stop being crabby? Why don't I choose kindness?
Internally, I do choose kindness. I want to be encouraging, tolerant, and a positive light in people's lives. I have already made this decision and am constantly thinking about how I want to be better. But that doesn't mean I always act kind, it just means I've taken the first step to being kind. It's important to mentally choose to be kind -- to reflect upon your day after it's over and think about how you were a good impact on others and how you weren't. I believe this is fundamental to being better because you get into the habit of recognizing when you are being a good influence and when you aren't, and become more likely to watch your words and actions before you do something.
The next part of being kinder is to take small steps. Smile at your friends even after you've had a rough day. Take a deep breath before you react negatively to a sibling who is getting on your nerves. Recognize when you're being petty and choose to let go of that bitterness. There have been countless times when I've chosen to remain bitter because I felt unsatisfied with the other person's lack of an apology, but of the few times I have let go of that feeling, I found myself feeling less annoyed with whoever bothered me and as though a weight was lifted off of my shoulders. Sometimes, you are the source of the weight on your shoulders. When you realize this, you will be able to think, "I'd rather be content than angry." and forgive the person who wronged you.
Being kind is a process. It takes times and it's about trial and error, but with every step (and misstep) you take, you get closer to automatically making kinder choices. It's a tedious process, but the end-game is so much sweeter than the satisfaction of being petty.