The only way that makes sense in my mind is if he gets them killed somehow and his response is to have a sort of mental pseudo-regeneration in order to help him move on from it. We've seen him spend plenty of time apart from them without any sort of personality crisis so it would need to be something big.The Doctor's going through a "Leisure Hive" type costume change
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There's a quote from Moffat that says the Eleventh Doctor hates his current look. He's trying to distance himself from Amy and Rory by changing clothes.
That quote sounds a bit off to me in general though. I'm dubious.