All suffering is caused by ignorance; people inflict pain on others in the selfish pursuit of their happiness or satisfaction.
Yet true happiness comes from a sense of peace and contentment, which in turn must be achieved through the cultivation of altruism, love and compassion, and the elimination of selfishness and greed.
The root of all our suffering in samsara is ignorance. Ignorance, until we free ourselves from it, can seem endless, and even when we have embarked on the spiritual path our search is fogged by it.
However, if you remember this, and keep the teachings in your heart, you will gradually develop the discernment to recognize the innumerable confusions of ignorance for what they are, and so never jeopardize your commitment or lose your perspective.
-Sangpo Rinpoche (reposted quote from Sanam Yalmo)