I've climbed up the side of F-4s a thousand times. You board aircraft from the left side because you mount horses from the left side. Horse are mounted from the left because cavalrymen wear their swords on their left, which makes it clumsy to mount from the right. Swords are carried on the left because most people are right-handed. People are right-handed because human hearts are on the left side. That means you need your left arm to shield your heart against a fatal blow. So, through the miracle of transitive logic, you can say you board aircraft from the left because your heart is on the left.