It's often an acknowledgement of appreciation. Either for being different (and recognized by another quirky person) or they are recognizing the work you have done. Or the fact that you never see motorized bicycles too often.
I have had a small number of sports cars. I would get some attention. Most of the attention I got was from dudes. Ironically, every girlfriend I have ever had I met while driving some small cheap little economy car.
My bikes are where I tend to get the most attention though. Custom paint jobs and a very cleaned and detailed ride always catches the eye. My 75 Goldwing for being a tasteful resto-mod of a classic. I would end up in conversations for hours with that thing. My VFR also gets a ton of looks, thankfully none from the 5-0 at the wrong time... while I am never ever speeding...