Traffic lights etiquette

Here's a situation where I find it difficult to keep my sense of good karma. You pull up to at a red traffic light to wait patiently for it to turn green. Another cyclist - and usually someone who is going at a very leisurely pace - pulls in right in front of you. Sure enough, when the light goes green, they amble through, blocking you as you try to get away with a slightly greater sense of urgency. What is it that stops people realising that they may be blocking their fellow cyclists? I'm quite happy for someone to go ahead of me if they believe they have a fair chance of going quicker ... but this. Still, if I'm to stick by my own rules, I have to quietly grin and bear it and say "good karma will come my way" ...