Why do people need to "warm up" around strangers?

[Stone the Crow]

That explains alot. So if i initiate contact, i am feeling like a superior bastard, and i must be treated like an obnoxious individual.

That response would suggest as much, seeing as how you lack the social and intellectual skills to fully understand what you read. But yes, in the end, most people will find you quite obnoxious (or at the very least, annoying) if you bother them at random just to engage in pointless small talk to make yourself more comfortable.

And yes, you are acting quite superior to everyone else based solely on your original post in the thread.

[Rizendragon]

Yeah I'm an ENTP. I love it when someone wants to talk to me on the plane, especially an attractive woman. If I don't feel like talking, I have no problem just saying, "I think I'm going to read my book now, if that's OK." People generally respect your wish to be allowed to do your thing once you state that preference.

I'm an INTP. Basically the only difference is that I take a bit longer to warm up before being wide-open. I'm more inclined to analyze the situation before opening up.

[Archonite]

Introverted people expend energy through social interaction, whereas extraverted people recharge through social interaction. As such, for an introvert, you dont want social energy you could be spending on your friends, family or loved ones, to be forcibly taken by you from strangers. You only have a limited amount of energy, and thus are more aware and cautious of strangers approaching you. Since extraverts often dont seem to understand this, they push past the boundaries of others without realizing, and tend to ignore any signs of discomfort from the other. The path of least resistance often leads to introverts tolerating extraverts until they get familiar enough to be considered worth the social energy. I will freely admit that introverts can sometimes/often need that push to build friendships and relationships, but it explains the initial cold attitude and needing to warm up.
As others have mentioned, confidence and anxiety also play a role. Out of all the tens or hundreds of people someone might meet on a day, the vast majority move by without interaction. Just by you starting to talk to them you show youre different, which doesnt have to be a good thing. People will question what your purpose or agenda is, are you trying to distract them and take their wallet, sign them up for a charity or try to sell them something? Since very few strangers actually approach me, quite a decent portion have been beggars, sales people and fundraisers. This depends on the situation ofcourse, if you meet new people at work, school or at a hobby club theyre much more likely to be actually interested in a conversation.