Short answer (and apologies to MetaFilter): babies are weird. Personality is something that starts developing from day one, and it is influenced by both genetic and environmental components. My daughter's a more apprehensive person than my son, and while generally happy as a baby, wasn't of the super-bubbly-isn't-life-grand outlook on life and could be VERY fussy (and at nearly 5, is better but still doesn't look at new situations with positive anticipation). My son was a "Gerber baby" in the parlance of our families; super cheerful, loved surprises and new things, would happily be held by anyone. He never went through the bad separation anxiety that she did, and is generally a more relaxed individual when it comes to new situations.
They're both super-healthy squirts, and get personality traits from both sides of the family. The daughter, while unhappy with changes to her schedule and routine, also has never met a stranger, which is TOTALLY like my father and TOTALLY unlike me and my husband. My son is rather unflappable, just like me, but can flip out when he's frustrated, just like his father. It's fun to see the different personality traits show up and realize just how individual each child is.