Our Shastras says "Buddhi Karmanusarani". If you break it up, it means like this "Buddhi + Karm + Anusarani" which means that our Buddhi i.e. intellect is guided as per our Prarabdha Karmas. Therefore, our past Karmas are the reasons behind the intellect taking the decision. If the Karma was good, our intellectual decision will reap positive results and if the Karmas was not good, our results will not be good.
The second Sutra is "Karma Buddhyanusarinam" which means our Kriyaman Karma which is done in the present moment is based on our Buddhi i.e. Intellect. When we understand both the sutras clearly, we come to the clear understanding that all our Intellectual decisions are based on our Past Karmas. A normal human being not following seriously the path of spirituality will be stuck in this vicious cycles which is called the Karma Bandhan i.e. bondage. This is the reason why "Enlightenment" is a rare commodity in today's world.
Until and unless you do not understand the Karma theory properly i.e. at our cellular level, we will be stuck in temporary happiness and sorrows of life.
Our Great Sages of the past had kept a well guarded secret called Karmakshalan Kriya. Very few Masters of today talk about this but this kriya helps you to come out of this Karmic Entanglement. There is a process for which we need to have the right attitude of learning to reach that stage. I have personally grown and experienced lot of changes in my life. Our Mind Intellect is not in a process to understand this knowledge, therefore, it would be better to experience it with an open mind.