The need to possess

Jealousy often expresses itself as a need to be the only person in another's life and to possess them, to the point where you deny them their right to independence.

The feeling of insecurity you experience with jealousy indicates your desire for a 'fusional' relationship, in which you are everything to each other - you're all he can see and he's all you can see. You ignore the fact that everyone needs their space - and we all know space is vital to any healthy relationship.