Jealousy slowly kills desire. If I take the example given in the first comment, imagine the girl who tells you that she finds you handsome (and is really sincere) and you start making up all sorts of scenarios that she might lie / might find somebody better because you see that she sees a lot of men and becomes suspicious of every comment she makes or every action she takes ; it shows in your non-verbal, it stresses the girl and she becomes distant because she feels observed and judged for every tiny detail, your experience is subsequently tinged by your own negative vibe that you yourself have established.
