People value their territorial bubble. Sometimes they think their bubble is bigger than it actually is, or make an effort to actively expand it. Some people may unintentionally expand it with their horrible body odor, whether or not this is ultimately a good thing for them depends on the type of person they are. Liquids on the seat can just be frightening sometimes. If there’s no visible drip from the ceiling you can only guess at what could have been the cause of it. It could be some spilled soda, and if it were something from a person chances are the seats next to it wouldn’t be taken up either.

The thing with people sitting in the middle of a seat depends on the body type of the person. It could either just be kind of natural if a person is particularly wide, or there are other reasons if a person isn’t wide that they could be sitting in the middle. First reason could be they have no regard for the people around them, or that there are two empty seats between two people and they don’t want to sit closely to either one of them. That situation only can happen in a train with long rows of seats which I don’t usually draw in my comic.