What does it mean if a guy says he wants to keep you on your toes?

lexipl13k
I have been dating this guy off and on for about four years for months at a time. We go our separate ways either because of me or him at different times. So today he told me he does certain things sometimes so that he can "Keep me on my toes", what does that mean?
What does it mean if a guy says he wants to keep you on your toes?
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