Possible Clues:

• Response to "Have a nice day!"
• 'I'm not the only one?'
• 'It's not just me?'
• ''Have a good day!'' reply
• "Have a nice day" response, and a literal hint to this puzzle's theme
• ''So I'm not the only one?''
• Essence of a Caesarean quote
• 'Et tu?'
• "I'm not the only one?"
• 'Right back atcha'