Flummoxed by Niceness

People are nice to me and my reaction isn't pleasure, appreciation, or even embarrassment. Nope, I become utterly baffled. If you think that sounds odd, try living with it.
It all seems to have started with the advent of the prosthetic eyes. Suddenly total strangers were doing weird things like not freaking out, not ignoring me, and even (gasp) being friendly. Coming away from a cafe counter, I might actually be happy instead of cringing. Once I even think I got special treatment not because I'm disabled but because the man was flirting with me. With that one, the earth shifted on its axis and I felt the ground move.
After years of being treated in a certain manner, I honestly have no idea how to assess what is happening. This might seem like a thing to appreciate given that I complain constantly about my frustration with people's less than stellar behavior. Maybe I'm distrustful by nature, but when people behave in inexplicable ways, I start to fret. What am I missing? . Will it last? If I don't understand it, how do I know everything will be alright?
I am expert at dissecting other's odd behavior able to discern nuances and glean motivation from an initial greeting lasting ten seconds. Vast experience has honed my instincts and I am able to know how I should react. When a person holds out their hand and I don't realize it until someone tells me, I know to make a joke to put the other person at ease. If wait staff speaks to my companion about me, I easily insert myself into the conversation in a way that forces the person to interact with me. These are reactions akin to the reflexive jerk of a knee when tapped by a rubber hammer.
In the past, pleasant behavior was easy for me to decipher: the person had prior experience with disabled folks, excellent customer service training, wanted to be friends, and/or rarely wished for something romantic. Everything was simple and clear because it happened less often and the behavior only had a handful of possible motivations.
In contrast, now people are friendly just because. Lacking experience with such behavior, I repeatedly try to fit actions into the categories I know but without success. Instead of letting the puzzle go unsolved, I keep going over it in my mind trying to find some clue I've overlooked.
Do you know how frustrating it can be to not understand why someone is being nice? I guess many of you are used to people behaving in such a manner. Maybe it's how you would feel if people suddenly began gazing over your right shoulder whenever they spoke to you. It feels that arbitrary and inexplicable.
I hope for the sake of my sanity that I learn to read niceness with the skill I can apply to odd behavior. Unfortunately that knowledge took years to acquire and I don't have the patience for that prolonged process again. I also lack the brashness of an early twentysomething that allowed me to directly ask people why they did what they did. I think I need a native guide or something. Volunteers?