Compassion is the motivator that gets us moving and keeps the process moving. It keeps our intention pure, the second factor of the Noble Eightfold Path is Right Thought or Intention. The Buddha states these as thoughts directed to renunciation, free from ill will, free from cruelty. Now in order to get Right Intention we have to develop compassion and lovingkindness, the unselfish emotions in our life.
Together with Wisdom and Right View, that is understanding the Law of Cause and Effect and the Four Noble Truths, these unselfish emotions direct our practice and make sure that our efforts are directed towards seeing Dukkha, seeing the cause of it and ending it.
Many other motivations may be misdirected. With Right View and the understanding of Cause and Effect linked with Right Intention, we can get our speech correct and therefore we can develop Right Speech, which is the next factor of the Noble Eightfold Path. In case anyone of you don't know the other five factors they are Right Action, Right Livelihood, Right Effort, Right Mindfulness and Right Concentration. In order to keep our practice directed towards the Four Noble Truths, Dukkha, seeing the cause of it and ending it, it is very important to develop compassion, because the usual response to seeing Dukkha is aversion and turning away, not wanting to see it, and therefore we will never be able to see the true cause of it and be able to heal it. So developing compassion is extremely important in order to reach the Truth. It can transform our usual response to Dukkha into a wise, unselfish response.