Joshua Jackson admits ''monogamy'' isn't easy.

The 'Affair' star, who has been dating German actress Diane Kruger for nine years, believes their relationship is still going strong because they still feel ''compelled'' to make it work.

The 36-year-old actor told Us Weekly magazine: ''Monogamy is hard. The equation for me these last nine years is to ask, 'Is this labour of love still something done freely?' If you're compelled to make it work, then you will.''

Despite dating the blonde beauty, 38, since 2006, the former 'Dawson's Creek' star previously admitted they are unlikely to ever tie the knot.

Asked if he believed in the idea of marriage, he said: ''Do I believe it exists? Yes. [But for me] No, I'm not religious.''

Diane, who was previously married to Guillaume Canet for five years until 2006, has also brushed off the idea in the past, but revealed she is eager to start a family with Joshua in Paris in the future.

Speaking about her love of the French capital, where she lived as a model when she was 16, she said: ''It's part of the deal. I come with Paris, there's no negotiating that.

''I see myself raising kids there. I would love that, having little French babies in Bonpoint.''

She added: ''What's important is the commitment. It's not the size of the ring you wear on your finger.''