I think part of it is the person, part of it is karma (my personal belief system deals a lot with karma), and part of it is hormones in a lot of cases
Though I dunno if a bad start to a day nessecarily means the entire day will be bad. My day started out awful today but I feel like while not all the bad things are resolved that overall it's been a good productive day.
I think that if nothing but bad things happen that counts as a bad day, but yeah bad and good depend on the individual most of the time. Though this morning I woke up with a headache, which I think most people would say isn't a great start. I'm feeling better now though.
I try to look at a good or bad day based on the overall karmic tally really.