Nasti buddhiryukasya na chayuktasya bhavana
Na chabhavayatah shantirashantasya kuto sukham (66)
A person whose mind and wisdom is not controlled cannot devote himself to God only. (My translation from Bengali, the originals is in Sanskrit, a language unfortunately almost Latin for me)
As per Geeta – A person whose mind and wisdom is not controlled cannot have a fixed target (being stable in God), so he fails to achieve peace and happiness.
When one’s mind and wisdom is not controlled he is unable to set his target on God, only God because his mind keeps diverting towards materialistic pleasures.
As his target is not fixed the thought of selfless work does not arises in his mind, heart- the determination that I will work without even thinking for the outcome evades him.
He is controlled by his desires, thus unable to do his duties without wavering, and therefore peace eludes him. Without doing one’s duty unwaveringly one can’t have peace.
A man without peace cannot be happy, no matter how much his work is rewarded he seeks more and more and happiness runs away from him.