It is because the vital energy was engaged in it. When the situation is over, the energy is released but then it is not directed towards anything meaningful. Besides when we invest lot of emotions into anything and then it ends abruptly or badly there is a sense of loss and the vital goes on a strike. It sulks and withdraws its energy from all effort leading to a state of blankness, dryness, lack of will and enthusiasm, depression. Then it begins to feel that perhaps it was better to continue as one was at least getting something instead of nothing. Thirdly, and paradoxically there is a hidden joy behind struggle that is lost with the end of the struggle. It is like the sudden exhaustion of the athlete running a long distance or a mountaineer struggling to climb a high peak feels utterly exhausted at the close of the journey rather than during the intense effort.
All this is ultimately due to the desire-self. The sense of emptiness passes away with time as new interests, activities, people, even new challenges replace the old ones and the vital energy gets engaged with them. Turn the vital energy towards the Divine service or get busy with work and pick up some new and challenging activity to channelise the vital energy and the joy and enthusiasm will return again.
Affectionately,
Alok Da