Why do I owe federal taxes?

According to HR block software I owe $1451 on my taxes. I haven't changed my W4 since employment starting in 2003 and taxes are always taken out every pay period and i'm still claiming single. I bought a house in 06 and since then i've always gotten a nice refund, mainly because of the mortgage interest. I have always been single until I got married march 2010. My wife didn't work the entire year and we don't have kids. Am I being severely penalized just for getting married or my wife not working or both or is there something odd about what the software calculated?