The mind is hard to check. It is swift and wanders at will. To control it is good. A controlled mind is conducive to happiness.
Dhammapada 3 : 35
The mind is defiled with the mental poisons of greed and attachment, anger and hatred, egoism and delusion.
Unless and until we tame the mind through sense restraint, meditation and contemplation, true happiness and peace will elude us.