Being left out is NOT fun and I've had some very good friends that left me out of things. It makes you feel like you nobody cares about you and your are all alone. Just put yourselves in their shoes, if you were at home bored and all your so called "friends" were having a good time without you. Just think about it the next time you leave someone out.
See, I kinda agree with this because yes, it does hurt but, there is always someone you will love more than another. And you have to accept that because everyone has favorites but you can't be hurt by it because you are someone's favorite, you might just not know who's in particular.