Strange social custom, primarily practiced by subcultures like retirees, motel employees, undergraduates, etc. In most cases, the motivation is catharsis. Whining about little things takes attention away from the big, scary parts of life. It helps you laugh at yourself, because it's impossible not to realize how silly you sound, going on and on about your minor dramas and tragedies. This is demonstrated by the fact that these sessions often end with a statement like: "Shut up, it's not a big deal," which is usually all that any participant truly wants to hear.

These dialogues also accomplish a more important end, that of bringing people closer together. It's much easier to open up to mere acquaintances about little problems than to share experiences that were deeply effecting, things that reveal information about who you are and why. It can serve to begin establishing trust between people. Knowing that someone is willing to listen to you, even when you're talking about nothing, makes you feel safe in telling them less superficial secrets and fears.

It should be noted that there is a large contingent of people who, for whatever reason, do not participate in this exercise and are sometimes irritated by it. What the difference between the two general categories is, I cannot really say. You could chalk it up to different ways of dealing with emotions, or different base levels of trust toward strangers.

As shared experiences go, bonding through bitching can be invaluable. To be able to talk about nonsense with someone, but look into their eyes and see real empathy and understanding, is a trait friendships often take years to acquire. If minor complaints bring this about faster, it's worth the social transgression.