The passion commonly acquires new force and violence in both cases.
The efforts, which the mind makes to surmount the obstacle, excite the spirits and inliven the passion.
Uncertainty has the same influence as opposition.

The agitation of the thought; the quick turns it makes from one view to another; the variety of passions, which succeed each other, according to the different views; All these produce an agitation in the mind, and transfuse themselves into the predominant passion.
There is not in my opinion any other natural cause, why security diminishes the passions, than because it removes that uncertainty, which encreases them.

The mind, when left to itself, immediately languishes; and in order to preserve its ardour, must be every moment supported by a new flow of passion.

For the same reason, despair, though contrary to security, has a like influence.
It is certain nothing more powerfully animates any affection, than to conceal some part of its object by throwing it into a kind of shade, which at the same time that it chews enough to pre-possess us in favour of the object, leaves still some work for the imagination.

Besides that obscurity is always attended with a kind of uncertainty; the effort, which the fancy makes to compleat the idea, rouzes the spirits, and gives an additional force to the passion.
As despair and security, though contrary to each other, produce the same effects; so absence is observed to have contrary effects, and in different circumstances either encreases or diminishes our affections.
The Duc de La Rochefoucault has very well observed, that absence destroys weak passions, but encreases strong; as the wind extinguishes a candle, but blows up a fire.