While “stir” has a variety of closely related meanings, I think “to rouse to activity, evoke strong feelings, and to call forth” are most applicable to the intended meaning in these verses. Even as God-loving and God-fearing Christians, it’s quite easy to become complacent and sometimes honestly downright lazy in our relationships toward others.
We certainly do not serve a stagnant God. He is always moving – in-flowing and out-flowing – and He expects His pinnacles of creation to do the same. We are made in His image which means we share His attributes of love in motion.
As brethren, we must look out for one another, admonishing and reminding each other of our calling in Christ, that we were created to perform good works prepared by God beforehand (see Ephesians 2:10). All of us can sometimes use a bit of urging down that path, a bit of stirring in our pot to keep things moving and prevent stagnation.
How do we do that? We spend time together, speaking edifying words and demonstrating good works toward one another and our neighbors. Humans assimilate and emulate what we see around us, both for good and for ill. It is for that reason we never reside in the presence of evil, but flee from it (1 Thessalonians 5:22) lest we risk seduction by the world and by the enemy. And it is for that reason we stir and are stirred to love and good works by gathering and celebrating God’s glory.
Lord, keep us reminded by Your Holy Spirit to fellowship for the right reason of stirring one another up in our faith, love, and good works that we might unceasingly give You honor and glory. In Jesus’ name, Amen.