Zizek on what causes desire:
The object of desire is simply the desired object: let’s say, in simple sexual terms, the person whom I desire. The object cause of desire, on the other hand, is that which makes me desire this person. And the two are not the same. Usually, we are not even aware of what was the object-cause of desire - it requires psychoanalysis to learn what, for example, made me desire that particular woman. This is something along the lines of what Freud already called the unary feature - and on which Lacan later developed a whole theory: i.e. some feature which triggers my desire in the other. And I think this is how one should read Lacan’s statement that there is no sexual relationship. This means precisely that it is never simply me and my partner. There is at the centre of any relationship the object-cause of desire.