You are afraid of rejection further down the line. If you control the emotions then you're safe. If you let other people in, you can't control the situation anymore.
You over-analyse everything turtle man,that comes through in almost all your questions and answers.Now that is,in many ways,a smart way to approach life - but does not always help when it comes to matters of the heart..
Letting people get close means you allow them to hurt you. So, it is a way of avoiding -more- pain.
I suspect this sharper awareness and focus has bought other factors to mind. it is however a guess
Well if you'd like to know, your not the only one with this problem. :)
It's safer that way. Don't let anyone get close and no one can rip your heart out ;-)