This is there normal mode of expression. They are not looking for help, maybe not even that much commiseration. They may not even realize how negatively they're coming across. It's a bad habit.
So if you're on good terms, you can remark that most of what they say is negative. If they're redeemable, they'll make an effort to change. If you're not on those terms, you'll have to ignore the litany and tune them out. Not even look at them or acknowledge them when they start on another complaint, and then only give them attention when they talk about something that impacts your work, or is positive in some way.