7 reasons:
1. The Sleeper. Wankers who get on at least 8 stops before the train hits civilisation hence getting a comfy seat and sleeping blissfully through their entire journey every morning (annoyingly awakening at their correct stop without fail), whilst you stand in someones armpit for an hour.
2. The Space Invader. Commuter who has no knowledge of personal social space and insists on forcing you into a corner of the carriage even if it's half empty.
3. The Picker: Is unaware of the public situation in which they are in, and proceeds to pick nose, ears, eyes or any other orifice that happens to take their fancy, in front of you, without a care.
4. The Breakfast Boy. Flatulent commuter who lets you know what he had for breakfast in a roundabout way. His identity is a mystery, but you know he is on the same carriage as you at least once a week.
5. The Breakfast Boy version 2. Commuter who is probably suffering from a cold and therefore insists on breathing with his mouth open all over you, giving you a nice mix of Golden Grahams and morning breath. No matter how many times you sigh loudly or cough politely he will not close up.
5. The Eyeliner - Inexperienced commuter who hasn't yet learned the etiquette of: 'all trying to face the same way when standing on the train'. Ignoring this rule results in having to stand face-to-face with a complete stranger for half an hour. This leads to uncomfortable periods of eye-to-eye contact, followed by uncomfortable periods of trying not to make eye-to-eye contact, followed by uncomfortable periods of staring hard at the floor.
6. The Chatter. Fine if they are talking to someone else but a pain in the butt if they try to talk to you. 'No I do not want to make small talk to you, a complete stranger, for half an hour, with every other person in the carriage 'listening in' on our dull, awkward, little conversation. Its seven in the morning and I am tired and irritable. So leave me alone'. Usually an old person. Note: try to avoid being too interesting as they can develop into a serial chatter(see below).
7. Serial Chatter - as with chatter except they learn which carriage you usually ride in.
I rest my case.