he comes from poverty she comes from wealth, her money made him start in politics, hence his endless gratitude for Claire.this was explained by David Fincher, It's very interesting hear the directors commentary of season 1, you learn a lot of things.

"I'm worried I'll never be able to watch Kevin Spacey or Robin Wright in anything else ever again without hoping they get heat-rayed into a pile of dust." it's called good acting. I'm sorry: outstanding acting.

and the show will play with an eventually Hillary 2016 run, analogizing Claire's own political aspirations and the fact that, even if Wright has denied it, the Underwoods had a lot in common with the Clintons.