Islam wants a person to be constantly reminded of the Hereafter. In this regard, we have been told by the Qur’ān at various places that the people who succeed in the Hereafter will be given their account in the right hand and those who are condemned and punished in the Hereafter will receive their reward in the left hand:
Then he who is given his record in his right hand will say: ‘Take this and read it. I always thought that [one day] I shall have to face my reckoning’. Consequently, he will be in a life of bliss in a lofty garden whose fruits will be hanging low -- Eat and drink to your heart’s content as reward for what you did in days gone by. And he who is given his record in his left hand will say: ‘Would that I had not been given my record nor known what my account was. Would that [my] death had been final’. (69:19-26)
In order that a person keep the ideal of receiving his account in the right hand as a constant target before him, the Prophet (sws) has asked his followers to start certain routines like eating, drinking water etc with the right hand. This is a symbolic guidance. One should therefore try to follow it as much as possible.
However, a person who is born left-handed is an exception. He is not required to change his ‘real’ hand. He will certainly receive his account in the right hand in the Hereafter if he has done good deeds.
Answered by Shehzad Saleem