Benji Madden admits Cameron Diaz is his muse

The Good Charlotte guitarist told PEOPLE.com: "I'm really happy right now."

Asked how his relationship with the actress has affected his music, Benji added: "It's the same for any man, no matter what they do. It drives you. All people, I think, react the same way to the same situations ... like we were saying with Joel being a father ... we all want to be the best we can be."

A source recently claimed the musician has "already discussed marriage" with his 41-year-old girlfriend.

The insider said: "It's early, but they have talked about marriage.

"Benji spends so much time at Cameron's that it seems he now kind of lives there. They look very happy together ... Cameron hasn't looked this happy for years."

Meanwhile, Benji's twin brother, Joel Madden, also says that his love for wife Nicole Richie and their children, Harlow, six, and Sparrow, four, has influenced his music.

He said: "I think it affects your life, and that affects your music. Ultimately, my life revolves around my family. Everything I do is for them. I want my family to be happy. And so that has been a life change over the last few years."