The reason is not simply because of my opinion of him. It is because it is unimaginable that you can stop the civil war there when the overwhelming majority of people in Syria consider him to be a brutal, murderous dictator.

Obama suggesting Bashar al-Assad must leave office to end the Syrian Civil War (19 November 2015)

Remarks by President Obama and Prime Minister Trudeau of Canada After Bilateral Meeting | whitehouse.gov[obamawhitehouse.archives.gov]