What makes you have them?
For me it is music, scents, and weather.
Apparently smell has the strongest memory.
I took the bus into town one day and a woman got on at one of the stops. She sat down on the seat in front of mine and something about the smell of her perfume was driving my homicidal urges fucking crazy. All I could think about for the rest of the trip was how much I wanted to pull her head back and slit her throat. It didn't just feel like a want, it felt like an intense need. The urge to do it was that strong my hands had literally begun to shake.
I was tempted to follow her when she got off the bus, but I didn't. My stop arrived before hers and I got off the bus instead.