Evolution of "push somebody's buttons" and "know what buttons to push"
It's a metaphorical usage, so there doesn't have to be a direct link between people and pushing the buttons they don't have.
Dictionary.com indicates the phrase originated in 1920s America, when domestic electricity was being installed on a massive scale. The ability to push a button to turn something on had a big impact on lifestyle, and the metaphor spilled over into other things which have an immediate effect — like being able to trigger a specific emotion.
During the 1920s the spread of technology transformed the way average Americans lived their daily lives. In 1920 only 34.7 percent of American dwellings had electricity; by 1930 67.9 percent had electric power. In the cities the growth was even more dramatic: 84.8 percent of all urban homes were wired for electricity by 1930, compared to only 47.4 percent a decade earlier.
At the turn of the century, families with a comfortable income depended on servants, hired labor, and delivery men to support their day-to-day lives. The new technology available in the 1920s—combined with a diminishing supply of people willing to work as servants—caused more and more middle-class fami-lies to do their own housework. By 1926 80 percent of American homes with incomes more than $3,000 had vacuum cleaners and washing machines.
American Decades © The Gale Group Inc
I can't understand how come "buttons" are associated with people and their behavior.
With machinary, you do one thing, and something else happens. Sometimes, the same things happens with people's emotions. You say the wrong words, and they react in a visible way. You show them something really cool, and they get excited.
The metaphors abound, and they're not limited to push his buttons. We also have:
- flipped his switch
- tripped his trigger
- turned his crank
- turned him off (or on)
Those are all cause-and-effect metaphors between machinary and people. (I'd guess there are others, too; those are just the ones I could think of off the top of my head.)
"Well the army never turned his crank / But love sure made him brave"
(from the song Duncan by Sarah Slean)