People could change for the better or for the worse. Today people may be good friends, tomorrow they could be bad enemies, or vice versa. In friendship and in hostility, people bless, as well as curse. Blessing and cursing is in the power of the tongue, but he/she who is able to tame his/her own tongue has wisdom and strength mightier than that of an army.
Therefore train your tongue such that in good times, while things are fine between you and other people, be quick to never miss the opportunity to bless and pray for them, so that when there will come a time or a situation where things turn bad between you and them, it would be too late for your cursing tongue to reverse the effect of the blessing and prayer you had given them.
Also, tame your tongue in such a way that in bad times, when things are not good between you and other people, and you can't find the strength to bless them, at least never say a curse against them. You will never know there will come a time when things could turn good between you and them, and it might be too late for your prayer to reverse the effect of the curse you had made against them.