A short blog post from Ideapod quotes Timothy Ferris:

“But you are the average of the five people you associate with most, so do not underestimate the effects of your pessimistic, unambitious, or disorganized friends. If someone isn’t making you stronger, they’re making you weaker.” ― Timothy Ferriss, The 4-Hour Workweek

The implication of this concept is–stop associating with people who are bringing you down!
There is a challenge to this from a missional or DMM standpoint: the people who need the Gospel are often these sorts of people (from one standpoint or another). They may be sinners – proud, angry, selfish, lust-filled, apathetic, gossiping, cruel. They may be sick or weak. They may be oppressed. They may be addicted.
The risk of the missional life is that we must associate with people who need the Gospel. People pick up the attitudes, values and behaviors of the people they hang around with. The friends of juvenile delinquents are at greater risk of juvenile delinquency.
But there is another way to think about it: if we are the average of the five people we associate with most, then we are also one of the five for someone who needs their average raised.
And if we can bring one or two other believers around a non-believer, so much the better. In fact, this is the way conversion typically works – it’s relational – people begin to trust the God their friends trust long before they ever make a “public confession” or get baptized.
“If someone isn’t making you stronger, they are making you weaker,” says one opinion. But, “God’s strength is perfect in our weakness,” is another. The way of the cross is the way of sacrifice, of decreasing, of weakness, of death, of giving up, of falling into the ground and dying–so that others may live. May we think about that more.