People can really irritate you when they don't live up to your expectations. Whether or not you realise it, the expectations we have of others maybe irrational or unrealistic. We expect people to behave in a specific way in certain situations, when they don't we usually become annoyed.
Sometimes it's ourselves being unreasonable, sometimes, it's the other person being unreasonable.
When you find yourself becoming aggravated by someone because they are not meeting your expectations, just stop for a moment and ask yourself, is it me being unreasonable, or is it them?