From a quote by "Claire" in the film Elizabethtown
The significant other of the substitute person
may not "have enough time" or "be too busy" for a relationship, or they
may only be interested for the purpose of sex, money, status, or general need of someone to talk to during times of stress. The substitute person generally won't get the emotional fulfillment from the experience that he/she
may crave, and will feel unimportant
yet somehow will continue on out of
hope.
This can only end in tears for the substitute person.