Ladies, what do you think of men that give you heads nods when you greet them or ignore your smiles?

My job combined with my home life and college creates lots of stress and anxiety for me, I have constant stimuli, excitement, and commotion coming at me from all angles. I am naturally more of an introverted shy person and rarely get to have peace. I still live at home and also have other brothers and sisters, I have my own space but there is constant noise and commotion. The only real time when I'm am able to get away from the madness is when I am outside of that realm Since I have no peace I make my own peace, I recede into myself and kind of live in my own little world. At school and work I am very quiet and dont talk much and kind of have a straight to the point no nonsense type of attitude. I am not a bad looking guy and I get approached by women all of the time I get smiles and greetings, but alot of the times I dont feel like talking or getting in long drawn out conversation beacuse of all thats going on in my life. I dont really like small talking and I dont feel like deciphering the jokes or sarcasm that is associated with the flirting. I also tend to keep a plain face, I work at a hospital so that makes it even worse. A women will approach me very happily with a big smile and say " Hello" I will reply with a simple hello, Hi, or headnod, they stand and look and look sad sometimes like I crushed them or they look offended like they expected someting more. It makes me feel bad sometimes, and I think why did I just do that her she was probably a nice and sincere person, my brain is racing so much at times it seems like a natural reflex to deflect people. Its hard some times because I know I miss out on alot of romantic oppurtunities and alot of women probably think I'm a jerk, but I cant help it. The hospital I work at is also a hostile and rude enviroment, and since I encounter so much rudeness and disrespect I think, why is this person just now coming out and being nice when everyone else is mean. Its all a big whirling pool of confusion. I also think alot of it has to do with my appearance, I'm tall brownish complexion about 6'2" 230 and I have an unfortunate baby face, long eyelashes big eyes, full lips and very soft features the way I feel inside doesn't really show on the outside, and I have a very soft and friendly appearance. I also tend to ignore them, but not intentionally. I am not gay or anything and I love women. Just wanted thoughts and insights mainly from women, but guys can answer as well.