Because uncontrolled, undisciplined mind is opposite of knowledge-wisdom and happy, that nature is dissatisfaction. So your mind put such way, control, with your own wisdom, there is space, you can discover peace and joyful. So your life is becomes joyful, peaceful, no matter the outside world is up and down, up and down.
In Los Angeles, California in 1975, Lama Yeshe gave this introductory talk on the purpose of meditation followed by an informal question and answer session. Please read along with the transcript on our website here: https://www.lamayeshe.com/article/chapter/purpose-meditation-part