In Washington D.C., people like power. They pretend they're in town to "serve" and "do the people's business" but really, they're there to line their pockets, enhance their post-office consulting fees, and savor the best scotch at all the right cocktail parties. There may be a few politicians (looking at you Paul, Cruz, and Amash) out there who actually vote their conscience and try to do the right thing but, by and large, pretty much all of them are in it for themselves.
When I was a child, I thought that attitude was cynical. That was a long time ago. These days I'm 100% certain that this is the only realistic lens through which you should ever view a career pol. They are in office to absorb power, access to power, and the money that comes with it.