Charles H. Spurgeon on Psalms 119:136

Rivers of waters run down mine eyes, etc. The Lord requireth this mourning bitterly for other men's sins to keep our hearts the more tender and upright; it is an act God useth to make us more careful of our own souls, to be troubled at the sins of others, at sin in a third person. It keepeth us at a great distance from temptation. This is like quenching of fire in a neighbour's house: before it comes near thee, thou runnest with thy bucket. There is no way to keep us free from the infection, so much as mourning. The soul will never agree to do that which it grieved itself to see another do. And, as it keepeth us upright, so also humble, fearful of Divine judgment, tender lest we ourselves offend, and draw down the wrath of God. He that shrugs when he seeth a snake creeping upon another, will much more be afraid when it cometh near to himself. In our own sins we have the advantage of conscience scourging the soul with remorse and shame; in bewailing the sins of others, we have only the reasons of duty and obedience. They that fight abroad out of love to valour and exploits, will certainly fight at home out of love to their own safety. Thomas Manton.