Ancient Scriptures have compared the mind to a monkey
not just any monkey but one which is drunk.
not just drunk but which has a thorn in its foot
not just thorn but possessed by evil spirits.
Now just imagine how erratic and unreasonable the monkey would behave.
Likewise our Mind vacillates without a moments rest from birth to Death.The Mind needs to be controlled and given rest to channelize its enormous energy in a constructive manner and this can be achieved only by Meditation.
In Bavagath Geeta,Arjuna complained about his vacillating mind to Lord Krishna and in response to that Krishna said it is the nature of ones mind to vacillate and anybody could control it by Meditation.He asked Arjuna not to get disheartened and told him to practice and can be achieved in course of time.