Here's an original idea, things come and go really fast. That was a lie by the way. But regardless, looking back is simple enough to do, something to remind yourself that there was a moment when your surroundings and the people who fill it up do not resemble how they are now. But what most people seem to forget is that this lack of resemblance isn't dramatic as they would wish. No these surroundings and those are people were the same, it's just you've grown old with them or grown out of them.
'Them' meaning either a surrounding that you possibly don't associate the same ideas or feelings with or meaning a person or a set of people whom you don't associate the same ideas or feelings with.
So what's the source of these changes in ideas and feelings? I'd say the amount of how many times you compare the past and present. If you look back long enough at a certain moment and compare it to the present there will result a drastic change in how you look at it. The new ideas and feelings that you've discovered in the present time frame will mesh with the old ideas and feelings you've had for a longer period of time, which 'updates' the way you see this moment in the past.
Time doesn't run out, it merely accumulates, and the more time you hold with an idea or a feeling the more you get to familiarize yourself with it's strengths and weaknesses. You grow comfortable with this and also grow sickened of how flawed it may be and the absurd nature to which you've devoted yourself to it.
I'd imagine that some memories are fondly remembered and no amount of future tarnishing can truly ruin it. The idea that it was a happy time is run by emotion and not reason, which allows one to look back and feel sorrow that it isn't like that anymore and wishing that your feelings and ideas were still valid or immediately relevant.
They are not immediately relevant because in your present mind you realize at some point that you can never achieve to re-live that preserved moment, it is impossible. But the relevance lives on in the individual that wishes to let it shape their future actions and creation of ideas on other new surroundings and their inhabitants.
In general terms, the only thing that really ever allows anything to change is you. Whether this can affect others and their ideas is up to themselves as well, which finally affects surroundings and gives birth to new ideas and feelings.