Motivation is the spark. A combination of enthusiasm and vision expected to lead to an internally satisfying reward.
Although this feeling is strong, it is not sustainable. Various biological (e.g. energy, health conditions), environmental factors (e.g. family, work pressure) can negatively contribute and significantly worsen motivation at certain times or periods of time.
Therefore, to keep the fire going, motivation is not enough. A deep sense of value and purpose and a high level of responsibility and discipline are required in order for the process that has begun to reach its intended outcome.
Good days will come, bad days will come, the effort never stops.