why do people do these things?

human behavīor is something hard to explain...

yesterday, we went to Red Robin for late lunch, early dinner... the poor waitress was having a rough time... there was two tables that was there to give her a bad time... she brought the burger to the lady... the lady complains, "OMG! there's cheese in it, I can't eat cheese!" so the waitress takes it back to the kitchen, brings her a new burger and some extra fries... after about 5 minutes, she complains again... "I can't eat this burger because you gave me all those fries, I'm stuffed." and she didn't want the burger anymore and wanted it off the tab... so the waitress took the burger away (and I assumed that she went to ask the manager about it.)

The manager comes out to talk to the lady and the lady went off and accused the poor waitress of a number of things... saying how she was rude and she wasn't attentive, blah blah blah... so the manager took the burger off the tab and asked the lady if there was something else he could get her... you know what the lady said? she requested for more fries... I think she ended up taking like 3-4 boxes of fries home... and that wasn't it... she complained that the soda was flat and ordered something else and said it wasn't cold enough, blah blah blah... *sigh*

then at another table, the similar situation happened... they didn't stop complaining from the moment they sat down... and in the end, they took home like 6 boxes of fries and cups of sodas...

I think for these two tables, there was like 5-6 people running around for them... *sigh* why do people do these things? do they wake up in the morning and just decides that they want to go out today and give everyone a hard time...