I used to milk cows and what it boils down to is that they just like to be milked, period. Cows produce enough milk to be milked twice and day, once every 12 hours, but to me, it seemed like a relief just to get it out. The cows I milked would produce anywhere from 1 to 4 gallons per milking session depending on age, the type of cow and the individual cow it’s self. The cows knew what time they were to be milked and would all gather around the barn at the same time everyday. We milked at 4AM and PM Even the new heifers would do it which makes me think that they wanted it out.
I can say this too, most of the cows would stay calm as long as they were eating the feed which lasted about the amount of time it took for the vacuum to milk them out but when you had to milk them out by hand, you hand to give them more feed or the would get cranky and start to kick at you because it took so much longer.