Animal behavior

Why does Riley bark at regular intervals (approximately every three seconds) when he wants to be let in, but Boscoe waits quietly, sometimes patiently lying down on the porch until we remember that he's outside? (You can tell by the above picture which one of them is the talky one.)

Why does Riley greet me when I come home by running right past me without a look until he secures the perimeter of the yard, then flings himself into my arms, while Boscoe greets me by seeing me, panicking, rushing deeper into the house for a puffy toy, and then rushing back up to me for ear scratches?

Why does Boscoe like being scratched on his head, but Riley likes being scratched on his butt?

Why is Riley standoffish but Boscoe is gregarious?

Why does Riley love pillows--he'll scratch around until he's made a deep nest of pillows and blankets and then bury himself in it to the point where you can barely see him--but Boscoe generally prefers the hard floor?

Why is Boscoe food-obsessed but Riley can kind of take it or leave it?

Why does Boscoe smile while Riley stares impassively?

Why, then, are they both afraid of water? Isn't Riley springer-Lab? Shouldn't he love water?

How is it that animals can have such distinct personalities? And how is it that we have learned to adapt ourselves wholeheartedly to them instead of expecting them to adapt themselves to us?