Instant relationships usually represent the "honeymoon phase" in a relationship. The time when everything great, because we are not interested in the obvious. There is a tendency to fall for the "potential of a relationship or person", rather than the "reality". The disappointment derives from two places, either you have emerged from the honeymoon and are seeing the obvious or you realize the person doesn/t feel the same intensity you do, which usually means you were impulsive and emotional naive. Relationships require time and commitment and you are fooling yourself when you think you can acheive anything meaningful from an instanteous encounter, where people are just discovering one another. Hope this helps!