Unsolicited conversations.

I ride the bus. I think the bus is great, although I refer to it as the looser cruiser. Having grown up, and lived in, towns with no public transportation and even no cabs I value this service. Now I know that any crazy can get on the bus, and many of these crazies will decide that they would like nothing better to chat or scream at me. When this gets to be too much I also enjoy walking and riding my bike, even drive sometimes. While I almost expect strangers to attempt a conversation on the bus I am still thrown off when strangers talk to me when I am in other transportation modes. Just the other day I was walking down the street. Along the way a man ended up right in front of me. Average guy jeans, shirt, about my age mid-day middle of the city. We both walked the same direction for a few blocks, when all of a sudden he stops turns and says, " great weather huh?" I look around and he seems to be speaking to me. I give him a look like he is clearly insane and proceed to cross the street and avoid this now identified weirdo. What is strange is that similar incidents keep occurring. While riding my bike I recently had an older seemingly normal woman ride up along side me and comment on how great a ride she was having. And of course on the bus strangers are starting conversations aplenty, but the people appear to be of sound mind. What is going on? Again coming from a small town I understand mindless banter and idle chit chat among strangers but why start a conversation with someone who is clearly not looking for one? On the bus I usually wear sunglasses or a hat, have headphones on and look straight ahead, a combination that I think conveys don't talk to me. I usually have the same thing going on when walking or riding my bike but I also am obviously moving which I thought would further deter conversation not invite. I don't necessarily mind these people talking to me, it does not hurt me but I never respond and some get annoyed. Why? What where you expecting a honest conversation that leads to good feelings and harmony among humanity? An exchange of ideas and perhaps friendship? Someone says hello or good day or gives the universal howdy head nod I usually can kick back the same in turn. however, anything beyond that you are out of luck. Why would you start a conversation with a stranger that does not seem open to your overture?