Nostalgia is delight from recalling the past.
The mechanism of delight produces some special situations.
One is nostalgia, a perk of delight's system. Nostalgia is a variation of delight.
We feel nostalgia when remembering events from our childhood for example. This is made by the system of delight when the present situation corresponds with a happy memory from the past. The difference is that the delight is from remembering the past, not a prediction of the future.
People feel nostalgia when they are reunited with a close childhood friend, or from seeing a memento. However, actions of nostalgia are generally weak in response to nostalgia being weaker than delight.
If a man and woman who were only acquaintances and not really friends when in school meet again as adults, they sometimes fall in love. This is because the delight of nostalgia strengthened their relationship.