What causes jealousy?

[Leviticus 26]

I've liked this guy for two years but most of the time my feelings do not result in jealousy. Like I remember a couple of weeks ago, I saw a guy who was wearing the same jacket as him holding a girl in his arms and wearing a knapsack. I thought it was him since they had the same height, built and body and I didn't even care... when I looked closer I realized that it wasn't him.

But now about 3 or 4 weeks later, I was agonizing over the prospect that the girl he was pursuing may have returned interested... I felt absolutely miserable when I looked at her facebook profile fearing the worst and discovered that she is still single and trying to attract guys which made me feel better.