The conjunction so
(Greek: διό/dio) indicates a logical connection between 1 Thessalonians 2:17–20 and 1 Thessalonians 3:1–5: Paul sent Timothy to the Thessalonians (1 Thessalonians 3:1–5) because he was deeply concerned for them yet unable to visit them in person (1 Thessalonians 2:17–20).1
1 Therefore when we could bear it no longer, we were willing to be left behind at Athens alone,