2 Thessalonians 3

"But as for you, brethren, do not grow weary in doing good." v.13

Read chapter 3
When does doing good exhaust you and why?
Paul knows the Thessalonians are doing good, but tells them to not grow weary in doing so.  He just finished talking about the lazies, those that are disorderly and busybodies so goes in to contrast of them that are not those things but to be aware of how those can cause weariness.  Sometimes when you're surrounded by others that are busybodies you get tired feeling all the work fall on you or you're doing all the work, so Paul says don't grow weary or exhausted, don't let that focus of others actions cause you to stop or wear out.  
He says in the verse before to work in quietness and eat their own bread.  "It is an excellent, but rare union, to be active in our own business, yet quiet as to other people's" (Matthew Henry's Commentary).
How can you not grow weary in doing good?  How can you avoid being brought down by those around you that are lazy, disorderly, or busybodies?