The 28-year-old is one of Arsenal's most influential players but he has failed to make an impact in recent weeks.
Arsenal manager Arsene Wenger even admitted he was struggling for form and confidence following the Gunners' 2-0 win over Hull.
His poor run of form comes after mounting speculation over his long-term future at the club, with the German yet to sign a new contract despite it expiring at the end of next season.
But former West Ham striker Ashton thinks Ozil's selfishness and moodiness could be the root cause of his problems.
Dean Ashton: Mesut Ozil does not look interested at Arsenal
“He can be absolutely unbelievable, and as ex-players we all look at Mesut Ozil and just think – wow!” Ashton told talkSPORT.
“But some days you can just see that he is not that interested.
“I think he’s probably one of those players who is a bit moody and you don’t really know what he’s thinking.
“Wenger probably looks at him and tries to get the best out of him, but he doesn’t give a lot away.
Ashton reckons Ozil get in moods if things don't go his way
Arsene Wenger has even admitted Ozil is struggling for form and confidence
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“He can get himself into one of those little moods if it’s not going his way.
“He makes it all about him, and therefore he’s forgetting what he needs to be doing for the team.”
Up next for Arsenal is a last-16 Champions League clash with Bayern Munich tomorrow at the Allianz Arena (7.45pm).