We tend to remember and sometimes even exaggerate the positive elements in our memories and forget about the negative ones, which is the idea behind the concept of rosy retrospection. The effect is more important for events that were slightly enjoyable when they occurred because the negative memories are forgotten much faster than positive ones. The problem with this bias is that it makes it hard for us to reconsider our actions after the fact and it might encourage us to repeat past actions that are not particularly enjoyable.
In one experiment, participants were asked to rate their vacations before, during and after having done the trip. The results show that they often had a positive anticipation before and a more positive recollection of the events after than what they though during the trip.