Why does my boyfriend leave bed to sleep on the couch?

Most nights my boyfriend of over 1.5 years will get up and go sleep on the couch. He says it's more comfortable but we have a brand new mattress. We've lived together for about 7 months and he's stayed over a lot before that. I can't recall him doing this until he moved in. It makes me feel like somethings wrong. I've tried to talk to him about it and tell him how it makes me feel but he insists it's nothing and continues to do it.
Why does my boyfriend leave bed to sleep on the couch?
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