I love Wikipedia, but sometimes it can be such an idiot. This is from the entry on Tom and Jerry:
The plots of each short usually center on Tom's (the cat) numerous attempts to capture Jerry (the mouse) and the mayhem and destruction that ensues. Since Tom rarely attempts to eat Jerry and because the pair actually seem to get along in some cartoon shorts it is unclear why Tom chases Jerry so much. Some reasons given may include normal feline/murine enmity, duty according to his owner, Jerry's attempt at ruining a task that Tom is entrusted with, revenge, Jerry saving other potential prey (such as ducks, canaries, or goldfish) from being eaten by Tom or competition with another cat, among other reasons.
Yes, it's a real puzzler, isn't it? Why does Tom (the cat) chase Jerry (the mouse) so much? Why aren't there more cartoon shorts in which Jerry (the mouse) helps Tom (the cat) with his tax return, or the pair of them do a crossword together? Why the constant chasing? Which of the some reasons given among other reasons can possibly explain it? I feel like we were almost on to something with 'normal feline/murine enmity', but... no, it's gone again. Just one of life's unsolved mysteries, I suppose. Maybe when Wikipedia has finally raised the six million dollars it keeps banging on about, we'll finally have the resources to work it out.