Why the Change?

Why is it that I can eat the same 5:00 o'clock snack for days in a row (most recently rye crisps with light paprika cream spread topped with smoked turkey breast) then wake-up one day and decide this snack no longer appeals to me? What made me change my mind? I suppose it's similar to how I never liked sparkling water (always thinking it tasted like flavorless soda), but one day, all that changed. Now I drink it everyday!