Jamieson, Fausset and Brown on Galatians 6:5

5. For (by this way, Ga 6:4, of proving himself, not depreciating his neighbor by comparison) each man shall bear his own "burden," or rather, "load" (namely, of sin and infirmity), the Greek being different from that in Ga 6:2. This verse does not contradict Ga 6:2. There he tells them to bear with others' "burdens" of infirmity in sympathy; here, that self-examination will make a man to feel he has enough to do with "his own load" of sin, without comparing himself boastfully with his neighbor. Compare Ga 6:3. Instead of "thinking himself to be something," he shall feel the "load" of his own sin: and this will lead him to bear sympathetically with his neighbor's burden of infirmity. ÆSOP says a man carries two bags over his shoulder, the one with his own sins hanging behind, that with his neighbor's sins in front.