Man, being a sociable creature, is mightily encouraged to do as others do, especially in an evil example; for we are more susceptible of evil than we are of good. Sickness is sooner communicated than health; we easily catch a disease one of another, but those that are sound do not communicate health to the diseased. – Thomas Manton
The importance of keeping with the right sort of friends is handily illustrated by Manton, a Puritan theologian who, whilst widely unknown, made significant contributions to Christian thinking about life and how believers ought to live it. If we keep the sort of friends who will lead us into harm then we will be led into harm. But if we keep the sort of friends who will help us behave properly, then we will behave properly.
Our tendency is to tell young people to choose their friends wisely but as adults we forget our own advice. Perhaps, then, we need to remember that we don’t catch health from the healthy but we do catch diseases from the diseased.