Menage a trois - One too many ?

Tweet

I started to think about relationships and people involved in them as I was walking pass a McDonald's downtown. Whenever you order a soda there they`ll always ask: Coke, Fanta, Sprite? So being in the dillema of choosing you are tempted to choose more than one. The same happens with fries: big,small, medium? (The size always tempts you ... again) So in a world full of temptation9s), is it just normal for us to want more when we consider that we have reached perfection ? I mean once in a relationship, why some want a 3rd person to make the "circle" complete? :) Once engaged in a relationship, what does the 3rd element mean?