Is it sinful to eat with the left hand for no reason?
It is Makruh (disliked) and should be avoided as mentioned in Sharh Tariqa Muhammadiyah of Imam Nabulusi.
Other Madhahib have narrations of it being Haram in which it will be a sin.
Allah knows best.
Answered by Shaykh Naveed Jameel ash-Shaami