The other afternoon I boarded a busy train full of people heading home after work. Although there were a few spare seats a number of passengers were standing.Three people got off at a nearby stop so it made sense for them to stand so they could alight easily. Others may just have avoided sitting because of the need for personal space.
Each person has their own feelings about personal space. There are those who hate sitting on a bench with someone on each side of them, their bodies touching. Some need more space around their body than others. This can be influenced by personality, culture, upbringing and past experiences.
As I made my way to a seat and wriggled myself in between two people who were seated I hoped I wasn't making either of them too uncomfortable by invading their personal space. I did want to sit down though and tried to take up as little space as possible.
People often feel uncomfortable emotionally as well as physically when a stranger, such as a fellow passenger on public transport, invades their space. It is impossible to avoid a degree of this when you are in a crowded place such as a train at peak hour.
There are others who don't understand the concept of personal space and they will stand or sit very close to another person without realising they are making him/her uncomfortable. Perhaps some realise but don't care.
In the situation of a person with autism or some cases of intellectual disability the person may not understand about personal space. A support worker or family member may be able to teach the concept but it is not always possible for the person to grasp it if they don't have a need for personal space themself.
There are individuals who are very uncomfortable with any form of touch. They may dislike handshakes, hugs or being kissed on the cheek, even by those they know well and are fond of. They aren't being rude when they shy away from touch, they are just very uncomfortable.
Then there are those people who feel a strong need to be physically close to others. They will also enjoy stroking someone on the hand or the arm and putting their arm around someone's shoulders. When the gesture is returned they will feel accepted and happy.
As with everything else, how much personal space a person needs is an individual matter. It is thoughtful to keep in mind the needs of the other person when moving near to them. If you notice body language that suggests you are too close, back off if possible.