Events lead you to people.
People lead you to places.
Places and people lead you to experiences.
Experiences in retrospection are memories.
As people fade away, and new ones meet, being nostalgic is the only justice that you can do to different sets of memories.
Time fades away, desires take new shapes. Dreams are redefined, and the individual is only a subset of the past.
New lives are born within the same body every day.
Some of the most nondescript things matter a lot. The smile of a face in the past, as the bus to the future carries you away, expectation of a phone-call although you have nothing to say, are not very significant things, but form an important part of nostalgia.
Life encourages you to improve, it allows you to explore more about yourself, and makes you strive for a unique level of freedom and comfort.
Collecting all experiences, so that maybe, I feel good while recollecting them in the future.