Why are acts of repentance especially appropriate during Lent?
Because it is the time leading up to the commemoration of Our Lord’s death for our sins and the commemoration of his resurrection for our salvation. It is thus especially appropriate to mourn the sins for which he died. Human have an innate psychological need to mourn tragedies, and our sins are tragedies of the greatest sort. Just as it is important to have set times in which to engage in rest and worship (e.g. Sunday), it is equally important to have set times of repentance- the season of Lent being one of those set times.