It is difficult to generalise such things and, i must say very very rare to move suddenly overnight. Generally a build up is taking place behind the facade of happy exterior, a growing inner unhappiness in the relationship for whatever reasons. During this phase one is quite vulnerable and has a tendency to shift grounds, withdraw or find a new partner. This certainly doesn’t happen overnight and if it does then it simply indicates that the person claiming to love was a big imposter.
As to words people say many things without meaning them often with ulterior motives of which sometimes even they may not be conscious as caught in a web of deceptive forces. That is why it has been said that do not trust the words blindly unless they are supported with actions that are commensurate. In any case once such a thing happens it is best not to get into a complaining mode. Instead of that one should reflect within and see what could have been charged from our side without blaming oneself or the partner and then go about one’s own life towards the future.
Affectionately,
Alok Da