Every human beings has a zone where they are alone. A small circle which is very personal, a space where noone will judge. This is the centre of a concentric circle. Beyond which we start allowing spaces to all the people we meet throughout life on the basis of how close we are to them and how much we relate to them. Exactly how electrons are arranged around the nucleus. There is a very specific and unique boundary for each one of the people that they are expected to respect. However human understanding of boundaries is very faint or I might be mistaken they don't acknowledge the existence of boundaries. Not always does a boundary has to be visible in red there are few which one should respect out of courtesy.
Beginning from that very young age when no one even considers the fact that even a child can draw lines which are not meant to be crossed to growing up into an insecure adult who is always scared of being violated, Indian households has seen it all.
I personally feel this should be taught at a very primary level understanding body languages and few very basic courtesies, so that one doesn't violate any just because they are ignorant. I'll just give an example, when someone is talking to you and moving towards you and you step back....Well this a sign that you drew a boundary: it's the distance you want to maintain with that person, it's a guarded response but humans are so ignorant they would continue to move forward.If you are sitting on a bench which is meant for two to sit down and you occupy the centre it's sign you want to sit "alone" now we all know a person who would squeeze in without asking. See I don't have any enmity with sitting down with people but I expect that one would atleast ask whether I want to share a bench or not. I mean sitting together when all the places are taken is different you don't get to choose but sitting together for the sake of sitting together should be a mutual decision. Peeping into someone's mobile phones I mean oh my god! Overhearing conversations( sometimes this is unavoidable I get it) but discussing them later is a big NO. Opening doors without knocking....gosh learn from Sheldon Cooper will you.
There are people who are tight lipped they won't even protest if they are feeling violated or not, it's a request please take care to understand if not then just directly ask before being impulsive. There are times one just wants to be alone it doesn't have to be because she is not your friend or doesn't want to be your friend. It's just that she wants to be in her "zone" i.e the innermost circle. These boundaries I am speaking of also include the type of questions you get to ask a person, even if you ask one and then feel that the person might not be comfortable answering it try to take a hint or change the topic. It's simple acts like this that can make you a very mature and understanding person.
This is to everyone make sure you have some lines drawn in black make sure they are justified and non-conpromisable by this I don't mean the petty ones like knocking on doors(I mean it's fact and no matter how serious you are about this no one cares I swear). Ones that should be valid , morally justified and serious and do not compromise with them because if you start bending lines to fit others whims one day looking back all you will see left are violations and nothing you can all yours.
This is no accusation but if we look back there will be times we all have violated boundaries too. But it's never too late to be a little concerned about the people around you and especially how you make them feel around you.
Having well defined boundaries doesn't mean you are introvert or a very private person...having boundaries is universal, trust me even very extroverted people has them too. Respect a healthy distance believe me it will make life easier of you can't identify it simply ask.