They spend their lives in small crates, and the females' only job is to produce more babies. Imagine the boredom the life of one pig could be, trapped in small crates and getting killed when they stop producing piglets. The sad thing about this whole issue is how unaware people are. When they are asked what they think happens to dairy cows once they get old, they say that they are probably given a field to graze in until they die, and then they would be eaten. These people are almost right, except for the part about getting a field to graze in before they die.
Some cows never see a field.
The first part of their lives is spent fattening them up for slaughter, and what they eat is directly fed to them in the small stalls that they live in, where they don't even have enough room to turn around. Sometimes they are given a field to graze in for the first months of their lives, but only for the purpose of fattening them up, and veal calves are never given the opportunity. There was once a contest to show people what it's like to live like a chicken. The were put in a relative size cage that a chicken would live in, so they had exactly the same amount of space. Recorded sounds of human laughter and screaming were constantly playing in the background.